Display device

ABSTRACT

A display device capable of operating at high speed and with low power consumption is provided. A miniaturized display device occupying a small area is also provided. The display device includes a support; a display portion which includes a pixel; a light-blocking unit which is in the support and includes a light-blocking layer having a first opening overlapping with at least part of the pixel, and a movable light-blocking layer blocking light passing through the first opening; a transistor which is electrically connected to the light-blocking unit and includes an oxide semiconductor film; and a capacitor electrically connected to the transistor.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a display device.

2. Description of the Related Art

Attention has recently been drawn to a display device using a mechanicalshutter, which is an application of micro electro mechanical systems(MEMS), or microstructures, and such a shutter is referred to as a MEMSshutter below. In the display device using a MEMS shutter, each pixel isprovided with a MEMS shutter which is opened and closed quickly with useof transistors so that images are displayed (for example, PatentDocument 1).

REFERENCE Patent Document [Patent Document 1]

-   Japanese Published Patent Application No. 2008-197668

SUMMARY OF THE INVENTION

In a mechanically operating display device such as the one using a MEMSshutter, high-speed and low-power operation is necessary to improveimage quality and reduce power consumption.

In view of the above, an object of one embodiment of the presentinvention is to provide a display device capable of operating at highspeed and with low power consumption.

Furthermore, miniaturization of each element is also required with anincrease in the integration density of a device. In view of this,another object of one embodiment of the present invention is to providea miniaturized display device occupying a small area.

In order to solve the above problems, in one embodiment of the presentinvention, a transistor including an oxide semiconductor is used forpart of a switching element which controls a light-blocking unit such asa MEMS shutter. In addition, a capacitor used for data storage is formedin manufacturing steps of the transistor. Such formation of thecapacitor makes a step over the capacitor more gradual; for example,part of a light-blocking unit can be overlapped with the capacitor,thereby obtaining a miniaturized display device occupying a small area.The structure will be described below in detail.

One embodiment of the present invention is a display device including asupport; a display portion which includes a pixel; a light-blocking unitwhich is in the support and includes a light-blocking layer having afirst opening overlapping with at least part of the pixel, and a movablelight-blocking layer blocking light passing through the first opening; afirst transistor which is electrically connected to the light-blockingunit and includes an oxide semiconductor film; and a capacitorelectrically connected to the first transistor. The capacitor includes afirst conductive film over the same surface as the oxide semiconductorfilm; an oxide insulating film which covers the first transistor andincludes a second opening on the first conductive film; a nitrideinsulating film which is over the oxide insulating film and in contactwith the first conductive film in the second opening; and a secondconductive film which is over the nitride insulating film andelectrically connected to the first transistor.

In the above structure, the first transistor includes a gate electrode,a gate insulating film in contact with the gate electrode, the oxidesemiconductor film in contact with the gate insulating film, and a pairof conductive films in contact with the oxide semiconductor film, andthe first conductive film is in contact with the gate insulating film.

In the above structure, the first conductive film and the oxidesemiconductor film include In, Ga, or Zn.

In the above structure, the light-blocking unit is a MEMS shutter.

The above structure further includes a second transistor which overlapswith and is electrically connected to the first transistor and thecapacitor, and the second transistor is provided on a substrateincluding a semiconductor material.

One embodiment of the present invention allows for providing a displaydevice capable of operating at high speed and with low powerconsumption, and also allows for providing a miniaturized display deviceoccupying a small area.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is an oblique drawing of a display device;

FIG. 2 is an isometric drawing of the display device;

FIG. 3 is a perspective view of a shutter in the display device;

FIG. 4 is a schematic view of a control circuit in the display device;

FIG. 5 is a cross-sectional view illustrating one mode of asemiconductor device;

FIGS. 6A to 6C are cross-sectional views illustrating one mode of amethod for manufacturing the semiconductor device;

FIGS. 7A to 7C are cross-sectional views illustrating one mode of amethod for manufacturing the semiconductor device;

FIGS. 8A to 8C are cross-sectional views illustrating one mode of amethod for manufacturing the semiconductor device;

FIGS. 9A and 9B are cross-sectional views illustrating one mode of amethod for manufacturing the semiconductor device;

FIGS. 10A to 10C are cross-sectional views illustrating one mode of amethod for manufacturing the semiconductor device;

FIGS. 11A and 11B are cross-sectional views each illustrating one modeof a transistor;

FIGS. 12A and 12B are cross-sectional views each illustrating one modeof a transistor;

FIG. 13 is a cross-sectional view illustrating one mode of a transistor;

FIG. 14 is a cross-sectional view illustrating one mode of a transistor;

FIG. 15 is a cross-sectional view illustrating one mode of a transistor;

FIG. 16 is a cross-sectional view illustrating one mode of a transistor;

FIG. 17 is a cross-sectional view illustrating one mode of asemiconductor device;

FIG. 18 is a cross-sectional view illustrating one mode of asemiconductor device;

FIGS. 19A to 19C each illustrate an electronic device using a displaydevice; and

FIGS. 20A to 20C illustrate an electronic device using a display device.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will be described in detail belowwith reference to drawings. Note that the present invention is notlimited to the following description, and it is easily understood bythose skilled in the art that various changes and modifications can bemade without departing from the spirit and scope of the presentinvention. Therefore, the present invention should not be construed asbeing limited to the description in the following embodiments. In thefollowing embodiments, the same portions or portions having similarfunctions are denoted by the same reference numerals or the samehatching patterns in different drawings, and description thereof willnot be repeated.

Note that in each drawing described in this specification, the size, thefilm thickness, or the region of each component is exaggerated forclarity in some cases. Therefore, the scale is not necessarily limitedto that illustrated in the drawings.

Terms such as “first”, “second”, and “third” in this specification areused in order to avoid confusion among components, and the terms do notlimit the components numerically. Therefore, for example, the term“first” can be replaced with the term “second”, “third”, or the like asappropriate.

Functions of a “source” and a “drain” are sometimes replaced with eachother when the direction of current flow is changed in circuitoperation, for example. Therefore, the terms “source” and “drain” can beused to denote the drain and the source, respectively, in thisspecification.

A voltage refers to a potential difference between two points, and apotential refers to electrostatic energy (electric potential energy) ofa unit charge at a given point in an electrostatic field. Note that ingeneral, a difference between a potential of one point and a referencepotential (e.g., a ground potential) is simply called a potential or avoltage, and a potential and a voltage are used as synonymous words inmany cases. Thus, in this specification, a potential may be rephrased asa voltage and a voltage may be rephrased as a potential unless otherwisespecified.

The term “electrically connected” includes the case where components areconnected through an “object having any electric function”. There is noparticular limitation on the “object having any electric function” aslong as electric signals can be transmitted and received between thecomponents connected through the object.

In this specification, the term “parallel” indicates that the angleformed between two straight lines is greater than or equal to −10° andless than or equal to 10°, and accordingly also includes the case wherethe angle is greater than or equal to −5° and less than or equal to 5°.In addition, the term “perpendicular” indicates that the angle formedbetween two straight lines ranges from 80° to 100°, and accordingly alsoincludes the case where the angle ranges from 85° to 95°.

Embodiment 1

In this embodiment, a semiconductor device of one embodiment of thepresent invention will be described with reference to drawings.

FIG. 1 is a schematic view illustrating a structure of a display deviceas an example of the semiconductor device. A display device 100illustrated in FIG. 1 includes a display portion 102 and a shutter-likelight-blocking unit 104.

The shutter-like light-blocking unit 104 allows switching between alight-blocking state and a transmission state. Note that thelight-blocking unit 104 may be any unit having a function of switchingbetween the light-blocking state and the transmission state; forexample, it may be a shutter including a light-blocking layer having anopening and a movable light-blocking layer capable of blocking lightpassing through the opening.

FIG. 2 is an isometric drawing showing the display device 100specifically. The display device 100 includes a plurality of supports106 a to 106 d (also collectively referred to as a support 106) whichare arranged in rows and columns. Each support 106 includes thelight-blocking unit 104 and an opening 112. The support 106 acorresponds to a pixel 102 a. Similarly, the supports 106 b to 106 dcorrespond to pixels 102 b to 102 d, respectively. The pixels 102 a to102 d constitute the display portion 102. The support 106 itself haslight-transmitting properties. When one or more of supports 106 havingspecific colors corresponding to the respective pixels are selectivelybrought into a transmission state, color pixels can be produced in thedisplay device 100.

The display portion 102 may be of a passive matrix type or an activematrix type; in the latter case, drive of elements is controlled bytransistors. In either case, wirings electrically connected to pixelsneed to be provided in a grid pattern. In order to improve apertureratio, the wirings in the display portion are preferably formed using aconductive film made of a light-transmitting conductive material.

When the display portion 102 is of the active matrix type, a transistoris preferably formed using a light-transmitting material. An oxidesemiconductor film is preferably used as a light-transmittingsemiconductor film of a transistor. Examples of the oxide semiconductorfilm include an In—Sn—Ga—Zn oxide, an In—Ga—Zn oxide, an In—Sn—Zn oxide,an In—Al—Zn oxide, a Sn—Ga—Zn oxide, an Al—Ga—Zn oxide, a Sn—Al—Znoxide, an In—Zn oxide, and a Sn—Zn oxide.

The light-blocking unit 104 is a MEMS shutter using MEMS technology. Thelight-blocking unit 104 includes a MEMS structure body and a MEMSdriving element. The MEMS structure body has a three-dimensionalstructure and includes a plurality of shutters which are partly movablemicrostructure bodies.

The MEMS structure body also includes, in addition to the light-blockinglayer and the movable light-blocking layer, an actuator for making themovable light-blocking layer slide parallel to the substrate surface, astructure body supporting the movable light-blocking layer, and thelike. An example of the structure of the MEMS shutter will be describedin detail later.

The MEMS driving element includes a transistor that drives the movablelight-blocking layer through the actuator. The transistor used in theMEMS driving element is preferably made of a light-transmitting materialand can be formed using a material similar to that of a transistor usedin the display portion 102. A conductive film used as a wiring in theMEMS driving element is preferably made of a light-transmittingconductive material.

Each support 106 is electrically connected to a scan line 114, a signalline 116, and a power source line 118. The light-blocking unit 104 isswitched between the light-blocking state and the transmission statedepending on potentials supplied from these lines.

Next, an example of the structure of the MEMS shutter that can be usedas the light-blocking unit 104 will be described with reference to FIG.3.

FIG. 3 illustrates a shutter 300. The shutter 300 includes a movablelight-blocking layer 302 bonded to an actuator 311. The actuator 311 isprovided over a light-blocking layer (not illustrated for simplicity)having an opening 304 and includes two flexible actuators 315. A side ofthe movable light-blocking layer 302 is electrically connected to theactuators 315. The actuators 315 have a function of moving the movablelight-blocking layer 302 in the direction of the line connecting astructure body 323 and a structure body 327.

The actuators 315 each include a movable electrode 321 electricallyconnected to the movable light-blocking layer 302 and a structure body319, and a movable electrode 325 electrically connected to the structurebody 323. The movable electrode 325 is adjacent to the movable electrode321. One end of the movable electrode 325 is electrically connected tothe structure body 323, and the other end thereof can be freely moved.The other end of the movable electrode 325 that can be moved freely iscurved so as to be closest to a connection portion of the movableelectrode 321 and the structure body 319.

The other side of the movable light-blocking layer 302 is connected to aspring 317 which returns to its original shape after force is applied bythe actuator 311. The spring 317 is connected to the structure body 327.

The structure bodies 319, the structure body 323, and the structure body327 function as mechanical supports to make the movable light-blockinglayer 302, the actuators 315, and the spring 317 float in the vicinityof the surface of the light-blocking layer having the opening 304.

Under the movable light-blocking layer 302, the opening 304 surroundedby the light-blocking layer is provided. Note that the shapes of themovable light-blocking layer 302 and the opening 304 are not limited tothese.

The structure body 323 included in the shutter 300 is electricallyconnected to a transistor (not illustrated). The transistor drives themovable light-blocking layer. Thus, a given voltage can be applied fromthe transistor to the movable electrode 325 connected to the structurebody 323. The structure bodies 319 and 327 are each connected to aground electrode (GND). Accordingly, the movable electrode 321 connectedto the structure body 319 and the spring 317 connected to the structurebody 327 each have a potential of GND. Note that the structure bodies319 and 327 may be electrically connected to a common electrode whichcan apply a given voltage. The structure bodies 319 and 327 may bereplaced with another actuator 311 so that the shutter includes the twoactuators 311.

When voltage is applied to the movable electrode 325, the movableelectrode 325 and the movable electrode 321 are electrically attractedto each other by a potential difference therebetween. As a result, themovable light-blocking layer 302 connected to the movable electrode 321is drawn toward the structure body 323 to move to the structure body323. Because the movable electrode 321 functions as a spring, when thepotential difference between the movable electrodes 321 and 325 iseliminated, the movable electrode 321 releases the stress accumulatedtherein so that the movable light-blocking layer 302 returns to itsoriginal position. In a state where the movable electrode 321 is drawnto the movable electrode 325, the movable light-blocking layer 302 mayblock the opening 304 or may be positioned so as not to overlap with theopening 304.

A method for manufacturing the shutter 300 will be described below. Asacrificial layer with a predetermined shape is formed by aphotolithography process over the light-blocking layer having theopening 304. The sacrificial layer can be formed using, for example, anorganic resin such as polyimide or acrylic, or an inorganic insulatingfilm such as silicon oxide, silicon nitride, silicon oxynitride, orsilicon nitride oxide. Note that in this specification and the like,silicon oxynitride contains more oxygen than nitrogen, and siliconnitride oxide contains more nitrogen than oxygen. The oxygen content andthe nitrogen content are measured by Rutherford backscatteringspectrometry (RBS) or hydrogen forward scattering spectrometry (HFS).

Next, a film of a light-blocking material is formed over the sacrificiallayer by a printing method, a sputtering method, an evaporation method,or the like and then is selectively etched, whereby the shutter 300 isformed. Examples of the light-blocking material include a metal such aschromium, molybdenum, nickel, titanium, copper, tungsten, tantalum,neodymium, aluminum, or silicon, and an alloy or an oxide thereof.Alternatively, the shutter 300 is formed by an inkjet method. Theshutter 300 is preferably formed to a thickness of 100 nm to 5 μm.

Then, the sacrificial layer is removed, whereby the shutter 300 whichcan be moved in a space can be formed. After that, a surface of theshutter 300 is preferably oxidized by oxygen plasma, thermal oxidation,or the like so that an oxide film is formed. Alternatively, aninsulating film of alumina, silicon oxide, silicon nitride, siliconoxynitride, silicon nitride oxide, DLC (diamond like carbon), or thelike is preferably formed on a surface of the shutter 300 by an atomiclayer evaporation method or a CVD method. Formation of the insulatingfilm on the shutter 300 can slow down the deterioration of the shutter300 over time.

Next, a control circuit 200 including the light-blocking unit will bedescribed with reference to FIG. 4.

FIG. 4 is a schematic view of the control circuit 200 in the displaydevice. The control circuit 200 controls the array of pixels in eachsupport 206 which includes a shutter provided with an actuator formaking a light-blocking unit in a light-blocking state and an actuatorfor making the light-blocking unit in a transmission state. The pixelsin the array each have a substantially square shape and the pitch, orthe distance between the pixels, is 180 μm to 200 μm.

In the control circuit 200, a scan line 204 is provided for the pixelsin each row, and a first signal line 208 a and a second signal line 208b are provided for the pixels in each column. The first signal line 208a supplies a signal for making the light-blocking unit in thetransmission state, whereas the second signal line 208 b supplies asignal for making the light-blocking unit in the light-blocking state.The control circuit 200 also includes a charge line 212, an operationline 214, and a common power source line 215. The charge line 212, theoperation line 214, and the common power source line 215 are sharedbetween the pixels in rows and columns of the array.

The support 206 including each of the pixels is electrically connectedto a transistor 216 charged to make the light-blocking unit in thetransmission state, and a transistor 218 discharged to make thelight-blocking unit in the transmission state. The transistor 218 iselectrically connected to a transistor 217 to which data is written sothat the light-blocking unit is brought into the transmission state, anda capacitor 219. The transistors 216 and 218 are electrically connectedto the actuator for making the light-blocking unit in the transmissionstate.

The support 206 including each of the pixels is also electricallyconnected to a transistor 220 charged to make the light-blocking unit inthe transmission state, and a transistor 222 discharged to make thelight-blocking unit in the transmission state. The transistor 222 iselectrically connected to a transistor 227 to which data is written sothat the light-blocking unit is brought into the transmission state, anda capacitor 229. The transistors 220 and 222 are electrically connectedto the actuator for making the light-blocking unit in the transmissionstate.

The transistors 216, 218, 220, and 222 include a material other than anoxide semiconductor material in a channel region, and therefore canoperate at sufficiently high speed.

The transistors 217 and 227 include a highly purified oxidesemiconductor in a channel region. When a transistor including a highlypurified oxide semiconductor in a channel region is turned off, data canbe retained in a floating node (e.g., a node at which the transistors217 and 218 and the capacitor 219 are connected, or a node at which thetransistors 222 and 227 and the capacitor 229 are connected). Inaddition, the transistor including a highly purified oxide semiconductorhas an extremely low off-state current, which eliminates the need for arefresh operation or significantly reduces the frequency of the refreshoperation, resulting in a sufficiently low power consumption.

The off-state current was actually measured using a transistor includingan oxide semiconductor and having a channel width W of 1 m. As a result,in the case where the drain voltage V_(D) is +1 V or +10 V and the gatevoltage V_(G) is in the range of −5 V to −20 V, the off-state current ofthe transistor was found to be lower than or equal to 1×10⁻¹² A which isthe detection limit, namely, lower than or equal to 1 aA (1×10⁻¹⁸ A) perunit channel width (1 μm). As the result of more accurate measurements,the off-state current at room temperature (25° C.) was lower than orequal to approximately 40 zA/μm (4×10⁻²⁰ A/μm) at a source-drain voltageof 4 V and lower than or equal to approximately 10 zA/μm (1×10⁻²⁰ A/μm)at a source-drain voltage of 3.1 V. Even at 85° C., the off-statecurrent was lower than or equal to approximately 100 zA/μm (1×10⁻¹⁹A/μm) at a source-drain voltage of 3.1 V.

It is thus confirmed that the off-state current of a transistorincluding a highly purified oxide semiconductor is sufficiently low. Fordetails about more accurate measurements of the off-state current,Japanese Published Patent Application No. 2011-166130 can be referredto.

A conductive film is formed on the same surface as an oxidesemiconductor film of the transistors 217 and 227 and used as oneelectrode of each of the capacitors 219 and 229. There is a small stepon the capacitors formed using such a conductive film, leading to easyintegration and miniaturization of the display device. For example, partof a light-blocking unit or a transistor can be overlapped with thecapacitor, thereby obtaining a miniaturized display device occupying asmall area.

In the control circuit 200, voltage is applied to the charge line 212first. Then, the transistors 216 and 220 are turned on because thecharge line 212 is connected to a gate and a drain of each of thetransistors 216 and 220. The minimum voltage needed to operate theshutter of the support 206 (e.g., 15 V) is applied to the charge line212. The charge line 212 is set to 0 V after charge of the actuator formaking the light-blocking unit in the light-blocking state and theactuator for making the light-blocking unit in the transmission state,whereby the transistors 216 and 220 are turned off. The charge in thetwo actuators is stored.

When a writing voltage V_(w) is supplied to the scan line 204, data issequentially written to the pixels in each row. During a period in whichdata is written to the pixels in a certain row, the control circuit 200applies a data voltage to one of the first signal line 208 a and thesecond signal line 208 b corresponding to each column of the pixels.When the voltage V_(w) is applied to the scan line 204 to which data isto be written, the transistors 217 and 227 in the corresponding row areturned on. When the transistors 217 and 227 are turned on, chargesupplied from the first signal line 208 a and the second signal line 208b is stored in the capacitors 219 and 229, respectively.

In the control circuit 200, the operation line 214 is connected to asource of each of the transistors 218 and 222. When the potential of theoperation line 214 is much higher than that of the common power sourceline 215, the transistors 218 and 222 are not turned off regardless ofthe charge stored in the capacitors 219 and 229. In the control circuit200, the transistors 218 and 222 are turned on/off depending on thecharge of the data stored in the capacitor 219 or 229 when the potentialof the operation line 214 is lower than or equal to that of the commonpower source line 215.

In the case where the transistor 218 or 222 is turned on, the charge ofthe actuator for making the light-blocking unit in the light-blockingstate or the charge of the actuator for making the light-blocking unitin the transmission state flows through the transistor 218 or 222. Forexample, when only the transistor 218 is turned on, the charge of theactuator for making the light-blocking unit in the transmission stateflows to the operation line 214 through the transistor 218. This causesa potential difference between the shutter of the support 206 and theactuator for making the light-blocking unit in the transmission state,so that the shutter is electrically attracted to the actuator and thetransmission state is obtained.

The cross-sectional view shown below is of a transistor which is used asa switching element in the control circuit 200 and of a capacitor inwhich charge can be stored. Here, the transistor 217 and the capacitor219, which are semiconductor devices in the control circuit 200, will bedescribed with reference to FIG. 5.

A gate electrode 404 of a transistor in a pixel portion is formed over asubstrate 402. There is no particular limitation on a material and thelike of the substrate 402 as long as the material has heat resistancehigh enough to withstand at least heat treatment performed later. Forexample, a glass substrate, a ceramic substrate, a quartz substrate, ora sapphire substrate may be used as the substrate 402. Alternatively, anSOI substrate or the like may be used and still alternatively, any ofthese substrates provided with a semiconductor element may be used asthe substrate 402. In the case where a glass substrate is used as thesubstrate 402, a large-sized liquid crystal display device can bemanufactured using a glass substrate with any of the following sizes:the 6th generation (1500 mm×1850 mm), the 7th generation (1870 mm×2200mm), the 8th generation (2200 mm×2400 mm), the 9th generation (2400mm×2800 mm), and the 10th generation (2950 mm×3400 mm).

Alternatively, a flexible substrate may be used as the substrate 402,and the transistor may be provided directly on the flexible substrate.Further alternatively, a separation layer may be provided between thesubstrate 402 and the transistor. The separation layer can be used whenpart or the whole of an element portion formed over the separation layeris completed and separated from the substrate 402 and transferred toanother substrate. In such a case, the transistor can be transferred toa substrate having low heat resistance or a flexible substrate as well.

For the gate electrode 404, a metal element selected from aluminum,chromium, copper, tantalum, titanium, molybdenum, and tungsten, an alloycontaining any of these metal elements as a component, an alloycontaining these metal elements in combination, or the like can be used.Alternatively, one or more metal elements selected from manganese andzirconium may be used. The gate electrode 404 may have a single-layerstructure or a layered structure of two or more layers. Examples of thestructure include a single-layer structure of an aluminum filmcontaining silicon, a two-layer structure in which a titanium film isstacked over an aluminum film, a two-layer structure in which a titaniumfilm is stacked over a titanium nitride film, a two-layer structure inwhich a tungsten film is stacked over a titanium nitride film, atwo-layer structure in which a tungsten film is stacked over a tantalumnitride film or a tungsten nitride film, a three-layer structure inwhich a titanium film, an aluminum film, and a titanium film are stackedin this order, and the like. Alternatively, a film, an alloy film, or anitride film which contains aluminum and one or more elements selectedfrom titanium, tantalum, tungsten, molybdenum, chromium, neodymium, andscandium may be used.

The gate electrode 404 can also be formed using a light-transmittingconductive material such as indium tin oxide, indium oxide containingtungsten oxide, indium zinc oxide containing tungsten oxide, indiumoxide containing titanium oxide, indium tin oxide containing titaniumoxide, indium zinc oxide, or indium tin oxide to which silicon oxide isadded. It is also possible to use a layered structure of the abovelight-transmitting conductive material and the above metal element.

An In—Ga—Zn-based oxynitride film, an In—Sn-based oxynitride film, anIn—Ga-based oxynitride film, an In—Zn-based oxynitride film, a Sn-basedoxynitride film, an In-based oxynitride film, a film of a metal nitride(such as InN or ZnN), or the like may be provided between the gateelectrode 404 and an insulating film 405 serving as part of a gateinsulating film. These films each have a work function 5 eV or higher,preferably 5.5 eV or higher, which is higher than the electron affinityof the oxide semiconductor. Therefore, the threshold voltage of thetransistor including an oxide semiconductor can be shifted in thepositive direction, and what is called a normally-off switching elementcan be achieved. For example, in the case where an In—Ga—Zn-basedoxynitride film is used, an In—Ga—Zn-based oxynitride film whosenitrogen concentration is higher than at least that of an oxidesemiconductor film 408 a, specifically, an In—Ga—Zn-based oxynitridefilm whose nitrogen concentration is higher than or equal to 7 at. % isused.

The insulating film 405 and an insulating film 406 are formed over thesubstrate 402 and the gate electrode 404. The insulating films 405 and406 serve as a gate insulating film of the transistor 217.

The insulating film 405 is preferably formed using a nitride insulatingfilm of silicon nitride, silicon nitride oxide, aluminum nitride, oraluminum nitride oxide, for example.

The insulating film 406 can be formed to have a single-layer structureor a layered structure using, for example, any of silicon oxide, siliconoxynitride, silicon nitride oxide, silicon nitride, aluminum oxide,hafnium oxide, gallium oxide, Ga—Zn-based metal oxide, and siliconnitride. The insulating film 406 may be formed using a high-k materialsuch as hafnium silicate (HfSi_(x)O_(y)), hafnium silicate to whichnitrogen is added, hafnium aluminate (HfAl_(x)O_(y)), hafnium aluminateto which nitrogen is added, hafnium oxide, or yttrium oxide, in whichcase the gate leakage current of the transistor can be reduced.

The total thickness of the insulating films 405 and 406 is greater thanor equal to 5 nm and less than or equal to 400 nm, preferably greaterthan or equal to 10 nm and less than or equal to 300 nm, and morepreferably greater than or equal to 50 nm and less than or equal to 250nm.

The oxide semiconductor film 408 a and a conductive film 408 b areformed over the insulating film 406. The oxide semiconductor film 408 ais formed to overlap with the gate electrode 404 and functions as achannel region. The conductive film 408 b functions as one electrode ofthe capacitor 219.

The oxide semiconductor film 408 a and the conductive film 408 b areeach typically an In—Ga oxide, an In—Zn oxide, or an In—M-Zn oxide (M isAl, Ti, Ga, Y, Zr, La, Ce, Nd, or Hf).

When the oxide semiconductor film 408 a and the conductive film 408 binclude an In—M-Zn oxide, the proportion of In and the proportion of M,not taking Zn and O into consideration, are preferably greater than orequal to 25 atomic % and less than 75 atomic %, respectively, morepreferably greater than or equal to 34 atomic % and less than 66 atomic%, respectively.

The energy gap of each of the oxide semiconductor film 408 a and theconductive film 408 b is greater than or equal to 2 eV, preferablygreater than or equal to 2.5 eV, and more preferably greater than orequal to 3 eV. The use of such an oxide semiconductor having a wideenergy gap reduces the off-state current of the transistor.

The thickness of each of the oxide semiconductor film 408 a and theconductive film 408 b is greater than or equal to 3 nm and less than orequal to 200 nm, preferably greater than or equal to 3 nm and less thanor equal to 100 nm, and more preferably greater than or equal to 3 nmand less than or equal to 50 nm.

The oxide semiconductor film 408 a and the conductive film 408 b can beformed using In—Ga—Zn oxide with an atomic ratio of In:Ga:Zn=1:1:1 or3:1:2. Note that the proportion of each atom in the atomic ratio of theoxide semiconductor film 408 a and the conductive film 408 b varieswithin a range of ±20% as an error.

Both the oxide semiconductor film 408 a and the conductive film 408 bare formed over the gate insulating film (here, over the insulating film406) but differ in impurity concentration. Specifically, the impurityconcentration in the conductive film 408 b is higher than that in theoxide semiconductor film 408 a. For example, the concentration ofhydrogen contained in the oxide semiconductor film 408 a is lower than5×10¹⁹ atoms/cm³, preferably lower than 5×10¹⁸ atoms/cm³, morepreferably lower than or equal to 1×10¹⁸ atoms/cm³, still morepreferably lower than or equal to 5×10¹⁷ atoms/cm³, and further morepreferably lower than or equal to 1×10¹⁶ atoms/cm³. The concentration ofhydrogen contained in the conductive film 408 b is higher than or equalto 8×10¹⁹ atoms/cm³, preferably higher than or equal to 1×10²⁰atoms/cm³, and more preferably higher than or equal to 5×10²⁰ atoms/cm³.The concentration of hydrogen contained in the conductive film 408 b isgreater than or equal to 2 times, preferably greater than or equal to 10times that in the oxide semiconductor film 408 a.

The conductive film 408 b has lower resistivity than the oxidesemiconductor film 408 a. The resistivity of the conductive film 408 bis preferably greater than or equal to 1×10⁻⁸ times and less than orequal to 1×10⁻¹ times the resistivity of the oxide semiconductor film408 a. The resistivity of the conductive film 408 b is typically greaterthan or equal to 1×10⁻³ Ωcm and less than 1×10⁴ Ωcm, preferably greaterthan or equal to 1×10⁻³ Ωcm and less than 1×10⁻¹ Ωcm.

When the oxide semiconductor film 408 a contains silicon or carbon whichis an element belonging to Group 14, the oxide semiconductor film 408 aincludes increased oxygen vacancies to have n-type conductivity.Therefore, the concentration of silicon or carbon (which is measured bysecondary ion mass spectrometry: SIMS) in the oxide semiconductor film408 a is set to lower than or equal to 2×10¹⁸ atoms/cm³, preferablylower than or equal to 2×10¹⁷ atoms/cm³.

The concentration of alkali metal or alkaline earth metal in the oxidesemiconductor film 408 a, which is measured by SIMS, is set to be lowerthan or equal to 1×10¹⁸ atoms/cm³, preferably lower than or equal to2×10¹⁶ atoms/cm³. Alkali metal and alkaline earth metal might generatecarriers when bonded to an oxide semiconductor, which may increase theoff-state current of the transistor. Therefore, it is preferable toreduce the concentration of alkali metal or alkaline earth metal in theoxide semiconductor film 408 a.

When containing nitrogen, the oxide semiconductor film 408 a is likelyto have n-type conductivity because of generation of electrons servingas carriers and an increase of carrier density. Thus, a transistorincluding an oxide semiconductor which contains nitrogen is likely to benormally on. For this reason, nitrogen in the oxide semiconductor filmis preferably reduced as much as possible; for example, theconcentration of nitrogen which is measured by SIMS is preferably set tobe lower than or equal to 5×10¹⁸ atoms/cm³.

An oxide semiconductor film with low carrier density is used as theoxide semiconductor film 408 a. For example, an oxide semiconductor filmwhose carrier density is lower than or equal to 1×10¹⁷/cm³, preferablylower than or equal to 1×10¹⁵/cm³, more preferably lower than or equalto 1×10¹³/cm³, and still more preferably lower than or equal to1×10¹¹/cm³ is used as the oxide semiconductor film 408 a.

Note that, without limitation to that described above, a material withan appropriate composition may be used depending on requiredsemiconductor characteristics and electrical characteristics (e.g.,field-effect mobility and threshold voltage) of a transistor. In orderto obtain required semiconductor characteristics of a transistor, it ispreferable that the carrier density, the impurity concentration, thedefect density, the atomic ratio of a metal element to oxygen, theinteratomic distance, the density, and the like of the oxidesemiconductor film 408 a be set to be appropriate.

The oxide semiconductor film 408 a is in contact with the insulatingfilms 406 and 412, which are formed using a material capable ofimproving the characteristics of the interface with the oxidesemiconductor film. Accordingly, the oxide semiconductor film 408 afunctions as a semiconductor, and the transistor including the oxidesemiconductor film 408 a has good electrical characteristics.

Note that it is preferable to use, as the oxide semiconductor film 408a, an oxide semiconductor film which has a low impurity concentrationand a low density of defect states, in which case the transistor canhave good electrical characteristics. Here, the state in which impurityconcentration is low and density of defect states is low (the amount ofoxygen vacancy is small) is referred to as “highly purified intrinsic”or “substantially highly purified intrinsic”. A highly purifiedintrinsic or substantially highly purified intrinsic oxide semiconductorhas few carrier generation sources, and thus has a low carrier densityin some cases. Thus, in some cases, a transistor including a channelregion formed in the oxide semiconductor film rarely has a negativethreshold voltage (is rarely normally-on). A highly purified intrinsicor substantially highly purified intrinsic oxide semiconductor film hasa low density of defect states and accordingly has few carrier traps insome cases. In addition, the highly purified intrinsic or substantiallyhighly purified intrinsic oxide semiconductor film has an extremely lowoff-state current; even when an element has a channel width of 1×10⁶ μmand a channel length (L) of 10 μm, the off-state current can be lessthan or equal to the measurement limit of a semiconductor parameteranalyzer, i.e., less than or equal to 1×10⁻¹³ A, at a voltage (drainvoltage) between a source electrode and a drain electrode of 1 V to 10V. Hence, the transistor in which the channel region is formed in theoxide semiconductor film has little variation in electricalcharacteristics and high reliability. Charges trapped by the trap statesin the oxide semiconductor film take a long time to be released and maybehave like fixed charges. Thus, the transistor in which the channelregion is formed in the oxide semiconductor film having a high densityof trap states has unstable electrical characteristics in some cases.Examples of the impurities include hydrogen, nitrogen, alkali metal,alkaline earth metal, and the like.

In an opening 462 (see FIG. 8A), the conductive film 408 b is in contactwith an insulating film 414 (see FIG. 8C) formed using a nitrideinsulating film. The insulating film 414 is made of a materialpreventing diffusion of impurities from the outside, such as water,alkali metal, and alkaline earth metal, into the oxide semiconductorfilm, and further includes hydrogen. Therefore, when hydrogen in theinsulating film 414 is diffused into the oxide semiconductor film formedat the same time as the oxide semiconductor film 408 a, hydrogen isbonded to oxygen and electrons serving as carriers are generated in theoxide semiconductor film. When the insulating film 414 is formed by aplasma CVD method or a sputtering method, the oxide semiconductor filmis exposed to plasma and oxygen vacancies are generated in the oxidesemiconductor film. When hydrogen contained in the insulating film 414enters the oxygen vacancies, electrons serving as carriers aregenerated. As a result, the oxide semiconductor film has higherconductivity and functions as a conductor. In other words, the oxidesemiconductor film can be referred to as an oxide semiconductor filmwith high conductivity or a metal oxide film with high conductivity.Here, a metal oxide film which mainly contains a material similar tothat of the oxide semiconductor film 408 a and has increasedconductivity is referred to as the conductive film 408 b.

Note that one embodiment of the present invention is not limitedthereto, and it is possible that the conductive film 408 b be not incontact with the insulating film 414 depending on circumstances.

Also, one embodiment of the present invention is not limited thereto,and the conductive film 408 b may be formed by a different process fromthat of the oxide semiconductor film 408 a depending on circumstances.In that case, the conductive film 408 b may include a different materialfrom that of the oxide semiconductor film 408 a. For example, theconductive film 408 b may be formed using indium tin oxide, indium oxidecontaining tungsten oxide, indium zinc oxide containing tungsten oxide,indium oxide containing titanium oxide, indium tin oxide containingtitanium oxide, indium tin oxide, indium zinc oxide, indium tin oxidecontaining silicon oxide, or the like.

In the semiconductor device shown in this embodiment, one electrode ofthe capacitor is formed at the same time as the semiconductor film ofthe transistor. In addition, the conductive film that serves as a pixelelectrode is used as the other electrode of the capacitor. Thus, a stepof forming another conductive film is not needed to form the capacitor,and the number of steps of manufacturing the semiconductor device can bereduced. Furthermore, the capacitor has light-transmitting propertiesbecause the pair of electrodes has light-transmitting properties. As aresult, the area occupied by the capacitor can be increased and theaperture ratio in a pixel can be increased.

A structure of the oxide semiconductor film will be described below.

An oxide semiconductor film is roughly classified into a single-crystaloxide semiconductor film and a non-single-crystal oxide semiconductorfilm. The non-single-crystal oxide semiconductor film includes any of anamorphous oxide semiconductor film, a microcrystalline oxidesemiconductor film, a polycrystalline oxide semiconductor film, a c-axisaligned crystalline oxide semiconductor (CAAC-OS) film, and the like.

The amorphous oxide semiconductor film has disordered atomic arrangementand no crystalline component. A typical example of the amorphous oxidesemiconductor film is an oxide semiconductor film in which no crystalpart exists even in a microscopic region, and the whole of the film isamorphous.

The microcrystalline oxide semiconductor film includes a microcrystal(also referred to as nanocrystal) with a size greater than or equal to 1nm and less than 10 nm, for example. Thus, the microcrystalline oxidesemiconductor film has higher degree of atomic order than the amorphousoxide semiconductor film. The density of defect states of themicrocrystalline oxide semiconductor film is therefore lower than thatof the amorphous oxide semiconductor film.

The CAAC-OS film is one of oxide semiconductor films including aplurality of crystal parts, and most of the crystal parts each fit intoa cube whose one side is less than 100 nm. Thus, there is a case where acrystal part included in the CAAC-OS film fits into a cube whose oneside is less than 10 nm, less than 5 nm, or less than 3 nm. The densityof defect states of the CAAC-OS film is lower than that of themicrocrystalline oxide semiconductor film. The CAAC-OS film is describedin detail below.

In a transmission electron microscope (TEM) image of the CAAC-OS film, aboundary between crystal parts, that is, a grain boundary is not clearlyobserved. Thus, in the CAAC-OS film, a reduction in electron mobilitydue to the grain boundary is less likely to occur.

According to the TEM image of the CAAC-OS film observed in a directionsubstantially parallel to a sample surface (cross-sectional TEM image),metal atoms are arranged in a layered manner in the crystal parts. Eachmetal atom layer has a morphology reflected by a surface over which theCAAC-OS film is formed (hereinafter, a surface over which the CAAC-OSfilm is formed is referred to as a formation surface) or a top surfaceof the CAAC-OS film, and is arranged in parallel to the formationsurface or the top surface of the CAAC-OS film.

On the other hand, according to the TEM image of the CAAC-OS filmobserved in a direction substantially perpendicular to the samplesurface (planar TEM image), metal atoms are arranged in a triangular orhexagonal configuration in the crystal parts. However, there is noregularity of arrangement of metal atoms between different crystalparts.

From the results of the cross-sectional TEM image and the planar TEMimage, alignment is found in the crystal parts in the CAAC-OS film.

A CAAC-OS film is subjected to structural analysis with an X-raydiffraction (XRD) apparatus. For example, when the CAAC-OS filmincluding an InGaZnO₄ crystal is analyzed by an out-of-plane method, apeak appears frequently when the diffraction angle (2θ) is around 31°.This peak is derived from the (009) plane of the InGaZnO₄ crystal, whichindicates that crystals in the CAAC-OS film have c-axis alignment, andthat the c-axes are aligned in a direction perpendicular to theformation surface or the top surface of the CAAC-OS film.

On the other hand, when the CAAC-OS film is analyzed by an in-planemethod in which an X-ray enters a sample in a direction perpendicular tothe c-axis, a peak appears frequently when 2θ is around 56°. This peakis derived from the (110) plane of the InGaZnO₄ crystal. Here, analysis(φ scan) is performed under the conditions where the sample is rotatedaround a normal vector of a sample surface as an axis (φ axis) with 2θfixed at around 56°. In the case where the sample is a single-crystaloxide semiconductor film of InGaZnO₄, six peaks appear. The six peaksare derived from crystal planes equivalent to the (110) plane. On theother hand, in the case of a CAAC-OS film, a peak is not clearlyobserved even when φ scan is performed with 2θ fixed at around 56°.

According to the above results, in the CAAC-OS film having c-axisalignment, while the directions of a-axes and b-axes are differentbetween crystal parts, the c-axes are aligned in a direction parallel toa normal vector of a formation surface or a normal vector of a topsurface. Thus, each metal atom layer which is arranged in a layeredmanner and observed in the cross-sectional TEM image corresponds to aplane parallel to the a-b plane of the crystal.

Note that the crystal part is formed concurrently with deposition of theCAAC-OS film or is formed through crystallization treatment such as heattreatment. As described above, the c-axis of the crystal is aligned in adirection parallel to a normal vector of a formation surface or a normalvector of a top surface. Thus, for example, in the case where the shapeof the CAAC-OS film is changed by etching or the like, the c-axis mightnot be necessarily parallel to a normal vector of a formation surface ora normal vector of a top surface of the CAAC-OS film.

Furthermore, the crystallinity in the CAAC-OS film is not necessarilyuniform. For example, in the case where crystal growth leading to theCAAC-OS film occurs from the vicinity of the top surface of the film,the crystallinity in the vicinity of the top surface is higher than thatin the vicinity of the formation surface in some cases. When an impurityis added to the CAAC-OS film, the crystallinity in a region to which theimpurity is added is changed, and the crystallinity in the CAAC-OS filmvaries depending on regions.

Note that when the CAAC-OS film with an InGaZnO₄ crystal is analyzed byan out-of-plane method, a peak of 2θ may also be observed at around 36°,in addition to the peak of 2θ at around 31°. The peak of 2θ at around36° indicates that a crystal having no c-axis alignment is included inpart of the CAAC-OS film. It is preferable that in the CAAC-OS film, apeak of 2θ appear at around 31° and a peak of 2θ do not appear at around36°.

A transistor including the CAAC-OS film has little variation inelectrical characteristics due to irradiation with visible light orultraviolet light. The transistor has high reliability accordingly.

Note that an oxide semiconductor film may be a stacked film includingtwo or more films of an amorphous oxide semiconductor film, amicrocrystalline oxide semiconductor film, and a CAAC-OS film, forexample.

The conductive films 410 a and 410 b are formed to have a single-layerstructure or a layered structure using, as a conductive material, any ofmetals such as aluminum, titanium, chromium, nickel, copper, yttrium,zirconium, molybdenum, silver, tantalum, and tungsten or an alloycontaining any of these metals as its main component. Examples of thestructure include a single-layer structure of an aluminum filmcontaining silicon, a two-layer structure in which a titanium film isstacked over an aluminum film, a two-layer structure in which a titaniumfilm is stacked over a tungsten film, a two-layer structure in which acopper film is formed over a copper-magnesium-aluminum alloy film, athree-layer structure in which a titanium film or a titanium nitridefilm, an aluminum film or a copper film, and a titanium film or atitanium nitride film are stacked in this order, and a three-layerstructure in which a molybdenum film or a molybdenum nitride film, analuminum film or a copper film, and a molybdenum film or a molybdenumnitride film are stacked in this order. Note that a transparentconductive material containing indium oxide, tin oxide, or zinc oxidemay be used.

The insulating films 412 and 414 are formed over the insulating film406, the oxide semiconductor film 408 a, the conductive film 408 b, andthe conductive films 410 a and 410 b. Like the insulating film 406, theinsulating film 412 is preferably formed using a material capable ofimproving the characteristics of the interface with the oxidesemiconductor film. The insulating film 412 can be formed using an oxideinsulating film. Here, the insulating film 412 includes a stack ofinsulating films 412 a and 412 b.

The insulating film 412 a is an oxide insulating film through whichoxygen is passed. Note that the insulating film 412 a also serves as afilm which relieves damage to the oxide semiconductor film 408 a and theconductive film 408 b at the time of forming the insulating film 412 blater.

As the insulating film 412 a, a silicon oxide film, a silicon oxynitridefilm, or the like with a thickness of 5 nm to 150 nm, preferably 5 nm to50 nm can be used.

It is preferable that the amount of defects in the insulating film 412 abe small, and typically the spin density corresponding to a signal whichappears at g=2.001 due to a dangling bond of silicon, be lower than orequal to 3×10¹⁷ spins/cm³ by ESR measurement. This is because if theinsulating film 412 a has a high density of defects, oxygen is bonded tothe defects and the amount of oxygen that permeates the insulating film412 a is decreased.

It is also preferable that the amount of defects at the interfacebetween the insulating film 412 a and each of the oxide semiconductorfilm 408 a and the conductive film 408 b be small, and typically thespin density corresponding to a signal which appears at g=1.93 due to andefect in the oxide semiconductor film 408 a and the conductive film 408b be lower than or equal to 1×10¹⁷ spins/cm³, more preferably lower thanor equal to the lower limit of detection by ESR measurement.

Note that all oxygen atoms entering the insulating film 412 a from theoutside are not moved to the outside of the insulating film 412 a andsome oxygen atoms remains in the insulating film 412 a in some cases. Inother cases, transfer of oxygen occurs in the insulating film 412 a insuch a manner that oxygen enters the insulating film 412 a and oxygencontained in the insulating film 412 a is moved to the outside of theinsulating film 412 a.

When an oxide insulating film which is permeable to oxygen is formed asthe insulating film 412 a, oxygen released from the insulating film 412b formed over the insulating film 412 a can be moved to the oxidesemiconductor film 408 a and the conductive film 408 b through theinsulating film 412 a.

The insulating film 412 b is formed in contact with the insulating film412 a. The insulating film 412 b is formed using an oxide insulatingfilm which contains oxygen at a higher proportion than thestoichiometric composition. Part of oxygen is released by heating fromthe oxide insulating film which contains more oxygen than that in thestoichiometric composition. In the oxide insulating film which containsoxygen at a higher proportion than the stoichiometric composition, theamount of released oxygen converted into oxygen atoms is greater than orequal to 1.0×10¹⁸ atoms/cm³, preferably greater than or equal to3.0×10²⁰ atoms/cm³ in TDS analysis.

As the insulating film 412 b, a silicon oxide film, a silicon oxynitridefilm, or the like having a thickness of 30 nm to 500 nm, preferably 50nm to 400 nm can be used.

It is preferable that the amount of defects in the insulating film 412 bbe small, and typically the spin density corresponding to a signal whichappears at g=2.001 due to a dangling bond of silicon, be lower than1.5×10¹⁸ spins/cm³, more preferably lower than or equal to 1×10¹⁸spins/cm³ by ESR measurement. Note that the defect density of theinsulating film 412 b may be higher than that of the insulating film 412a because the insulating film 412 b is more apart from the oxidesemiconductor film 408 a and the conductive film 408 b than theinsulating film 412 a is.

When the nitride insulating film having a blocking effect againstoxygen, hydrogen, water, alkali metal, alkaline earth metal, and thelike is provided as the insulating film 414, it is possible to preventoutward diffusion of oxygen from the oxide semiconductor film 408 a andthe conductive film 408 b. The nitride insulating film is formed usingsilicon nitride, silicon nitride oxide, aluminum nitride, aluminumnitride oxide, or the like.

Over the nitride insulating film having a blocking effect againstoxygen, hydrogen, water, alkali metal, alkaline earth metal, and thelike, an oxide insulating film having a blocking effect against oxygen,hydrogen, water, and the like may be provided. Examples of the oxideinsulating film having a blocking effect against oxygen, hydrogen,water, and the like include aluminum oxide, aluminum oxynitride, galliumoxide, gallium oxynitride, yttrium oxide, yttrium oxynitride, hafniumoxide, and hafnium oxynitride. In order to control the charge capacityof the capacitor, a nitride insulating film or an oxide insulating filmmay be further provided over the nitride insulating film having ablocking effect against oxygen, hydrogen, water, alkali metal, alkalineearth metal, and the like, as appropriate.

A conductive film 416 is formed over the insulating film 414. Theconductive film 416 is electrically connected to the conductive film 410b through an opening 464 (see FIG. 8C) and serves as a pixel electrodeof a pixel. The conductive film 416 can also function as one electrodeof the capacitor.

The conductive film 416 can be formed using a light-transmittingconductive material such as indium oxide including tungsten oxide,indium zinc oxide including tungsten oxide, indium oxide includingtitanium oxide, indium tin oxide including titanium oxide, indium tinoxide (ITO), indium zinc oxide, or indium tin oxide to which siliconoxide is added.

An alignment film 418 can be formed using an organic resin such aspolyimide. The thickness of the alignment film 418 is preferably greaterthan or equal to 40 nm and less than or equal to 100 nm, more preferablygreater than or equal to 50 nm and less than or equal to 90 nm. Withsuch a thickness, the pretilt angle of a liquid crystal material can bemade large, which can reduce disclination.

A liquid crystal element 422 is sandwiched between a pair of substrates(the substrate 402 and a substrate 442).

The liquid crystal element 422 includes the conductive film 416 over thesubstrate 402, the alignment film 418, an alignment film 452, a liquidcrystal layer 420, and a conductive film 450. The conductive film 416having light-transmitting properties serves as one electrode of theliquid crystal element 422, and the conductive film 450 serves as theother electrode of the liquid crystal element 422. Although notillustrated, a transistor for driving the liquid crystal element 422 isseparately provided.

A film having a coloring property (hereinafter referred to as a coloringfilm 446) is formed on the substrate 442. The coloring film 446functions as a color filter. In addition, a light-blocking film 444adjacent to the coloring film 446 is formed on the substrate 442. Thelight-blocking film 444 functions as a black matrix. The coloring film446 is not necessarily provided in the case where the liquid crystaldisplay device is a monochrome display device, for example.

The coloring film 446 is a coloring film that transmits light in aspecific wavelength range. For example, a red (R) color filter fortransmitting light in a red wavelength range, a green (G) color filterfor transmitting light in a green wavelength range, or a blue (B) colorfilter for transmitting light in a blue wavelength range can be used.

The light-blocking film 444 preferably has a function of blocking lightin a particular wavelength region, and can be a metal film or an organicinsulating film including a black pigment.

An insulating film 448 is formed on the coloring film 446. Theinsulating film 448 functions as a planarization layer or suppressesdiffusion of impurities in the coloring film 446 to the liquid crystalelement side.

The conductive film 450 is formed on the insulating film 448. Theconductive film 450 serves as the other of the pair of electrodes of theliquid crystal element in the pixel portion. Note that an alignment film418 is formed over the conductive film 416 and the alignment film 452 isformed on the conductive film 450.

The liquid crystal layer 420 is formed between the conductive film 416and the conductive film 450. The liquid crystal layer 420 is sealedbetween the substrate 402 and the substrate 442 with the use of asealant (not illustrated). The sealant is preferably in contact with aninorganic material to prevent entry of moisture and the like from theoutside.

A spacer may be provided between the conductive film 416 and theconductive film 450 to maintain the thickness of the liquid crystallayer 420 (also referred to as a cell gap).

A method for manufacturing an element portion over the substrate 402 inthe liquid crystal display device illustrated in FIG. 5 will bedescribed with reference to FIGS. 6A to 6C, FIGS. 7A to 7C, FIGS. 8A to8C, and FIGS. 9A and 9B. Here, the element portion over the substrate402 refers to a region sandwiched between the substrate 402 and thealignment film 418.

First, the substrate 402 is prepared. Here, a glass substrate is used asthe substrate 402.

Next, a conductive film is formed over the substrate 402 and processedinto desired regions, so that the gate electrode 404 is formed. The gateelectrode 404 can be formed in such a manner that a mask is formed inthe desired regions by first patterning and regions not covered with themask are etched (see FIG. 6A).

The gate electrode 404 can be typically formed by an evaporation method,a CVD method, a sputtering method, a spin coating method, or the like.

Next, the insulating film 405 is formed over the substrate 402 and thegate electrode 404, and then the insulating film 406 is formed over theinsulating film 405.

The insulating films 405 and 406 can be formed by a sputtering method, aCVD method, or the like. Note that it is preferable that the insulatingfilms 405 and 406 be formed in succession in a vacuum, in which caseentry of impurities is suppressed.

Next, an oxide semiconductor film 407 is formed over the insulating film406 (see FIG. 6B).

The oxide semiconductor film 407 can be formed by a sputtering method, acoating method, a pulsed laser deposition method, a laser ablationmethod, or the like.

Next, the oxide semiconductor film 407 is processed into desiredregions, so that the oxide semiconductor film 408 a and an oxidesemiconductor film 408 c having island-like shapes are formed. The oxidesemiconductor films 408 a and 408 c can be formed in such a manner thata mask is formed in the desired regions by second patterning and regionsnot covered with the mask are etched. For the etching, dry etching, wetetching, or a combination of dry etching and wet etching can be employed(see FIG. 6C).

After that, heat treatment may be performed so that hydrogen, water, andthe like included in the oxide semiconductor films 408 a and 408 c arereleased to reduce the concentrations of hydrogen and water in the oxidesemiconductor films 408 a and 408 c. As a result, highly purified oxidesemiconductor films 408 a and 408 c can be formed. The heat treatment isperformed typically at a temperature of 250° C. to 650° C., preferably300° C. to 500° C. When the heat treatment is performed typically at atemperature of 300° C. to 400° C., preferably 320° C. to 370° C., warpor shrinking of a large-sized substrate can be reduced to improve yield.

An electric furnace, an RTA apparatus, or the like can be used for theheat treatment. With the use of an RTA apparatus, the heat treatment canbe performed at a temperature higher than or equal to the strain pointof the substrate if the heating time is short. This leads to shorteningof the heat treatment time and reduces warp of the substrate during theheat treatment, which is particularly advantageous to a large-sizedsubstrate.

The heat treatment may be performed under an atmosphere of nitrogen,oxygen, ultra-dry air (air with a water content of 20 ppm or less,preferably 1 ppm or less, and more preferably 10 ppb or less), or a raregas (argon, helium, or the like). The atmosphere of nitrogen, oxygen,ultra-dry air, or a rare gas preferably does not contain hydrogen,water, and the like. After heat treatment performed in a nitrogenatmosphere or a rare gas atmosphere, heat treatment may be additionallyperformed in an oxygen atmosphere or an ultra-dry air atmosphere. As aresult, hydrogen, water, and the like can be released from the oxidesemiconductor film and oxygen can be supplied to the oxide semiconductorfilm at the same time. Consequently, the amount of oxygen vacancies inthe oxide semiconductor film can be reduced.

Next, a conductive film 409 is formed over the insulating film 406, theoxide semiconductor film 408 a, and the oxide semiconductor film 408 c(see FIG. 7A).

The conductive film 409 can be formed by a sputtering method, forexample.

Then, the conductive film 409 is processed into desired regions, so thatthe conductive films 410 a and 410 b are formed. The conductive films410 a and 410 b can be formed in such a manner that a mask is formed inthe desired regions by third patterning and regions not covered with themask are etched (see FIG. 7B).

Next, an insulating film 411 including a stack of an insulating film 411a and an insulating film 411 b is formed to cover the insulating film406, the oxide semiconductor film 408 a, the oxide semiconductor film408 c, the conductive film 410 a, and the conductive film 410 b (seeFIG. 7C).

Note that after the insulating film 411 a is formed, the insulating film411 b is preferably formed in succession without exposure to the air.After the insulating film 411 a is formed, the insulating film 411 b isformed in succession by adjusting at least one of the flow rate of asource gas, pressure, a high-frequency power, and a substratetemperature without exposure to the air, whereby the concentration ofimpurities attributed to the atmospheric component at the interfacebetween the insulating film 411 a and the insulating film 411 b can bereduced and oxygen in the insulating film 411 b can be moved to theoxide semiconductor films 408 a and 408 c, reducing the amount of oxygenvacancies in the oxide semiconductor films 408 a and 408 c.

As the insulating film 411 a, a silicon oxide film or a siliconoxynitride film can be formed under the following conditions: thesubstrate placed in a treatment chamber of a plasma CVD apparatus thatis vacuum-evacuated is held at a temperature higher than or equal to180° C. and lower than or equal to 400° C., preferably higher than orequal to 200° C. and lower than or equal to 370° C., the pressure in thetreatment chamber is greater than or equal to 20 Pa and less than orequal to 250 Pa, preferably greater than or equal to 100 Pa and lessthan or equal to 250 Pa with introduction of a source gas into thetreatment chamber, and a high-frequency power is supplied to anelectrode provided in the treatment chamber.

A deposition gas containing silicon and an oxidizing gas are preferablyused as the source gas of the insulating film 411 a. Typical examples ofthe deposition gas containing silicon include silane, disilane,trisilane, and silane fluoride. Examples of the oxidizing gas includeoxygen, ozone, dinitrogen monoxide, and nitrogen dioxide.

Under the above conditions, an oxide insulating film which is permeableto oxygen can be formed as the insulating film 411 a. In addition, byproviding the insulating film 411 a, damage to the oxide semiconductorfilms 408 a and 408 c can be reduced in a later step of forming theinsulating film 411 b.

Under these film formation conditions, the bonding strength of siliconand oxygen becomes strong when the substrate temperature is thedeposition temperature of the insulating film 411 a. Thus, as theinsulating film 411 a, a dense and hard oxide insulating film which ispermeable to oxygen, typically, a silicon oxide film or a siliconoxynitride film of which etching using hydrofluoric acid of 0.5 wt % at25° C. is performed at a rate of 10 nm/min or lower, preferably 8 nm/minor lower can be formed.

The insulating film 411 a is formed while heating is performed; as aresult, hydrogen, water, or the like contained in the oxidesemiconductor films 408 a and 408 c can be released in the step.

In addition, since heating is performed in the step of forming theinsulating film 411 a, the exposed oxide semiconductor films 408 a and408 c are not subjected to heating for a long time. This reduces theamount of oxygen released from the oxide semiconductor films by heattreatment. That is, the amount of oxygen vacancies in the oxidesemiconductor films can be reduced.

Furthermore, by setting the pressure in the treatment chamber to begreater than or equal to 100 Pa and less than or equal to 250 Pa, theamount of water contained in the insulating film 411 a is reduced; thus,variation in electrical characteristics of the transistor can be reducedand change in threshold voltage can be inhibited.

Moreover, by setting the pressure in the treatment chamber to be greaterthan or equal to 100 Pa and less than or equal to 250 Pa, damage to theoxide semiconductor films 408 a and 408 c can be reduced when theinsulating film 411 a is formed, resulting in a reduced amount of oxygenvacancies contained in the oxide semiconductor films 408 a and 408 c. Inparticular, when the film formation temperature of the insulating film411 a or the insulating film 411 b which is formed later is set to behigh, typically higher than 220° C., part of oxygen contained in theoxide semiconductor films 408 a and 408 c is released so that oxygenvacancies are easily formed. In addition, when the film formationconditions for reducing the amount of defects in the insulating film 411b which is formed later are used to increase the reliability of thetransistor, the amount of released oxygen is likely to be reduced. Thesemake it difficult to reduce oxygen vacancies in the oxide semiconductorfilms 408 a and 408 c in some cases. However, by setting the pressure inthe treatment chamber to be greater than or equal to 100 Pa and lessthan or equal to 250 Pa to reduce damage to the oxide semiconductorfilms 408 a and 408 c at the time of forming the insulating film 411 a,oxygen vacancies in the oxide semiconductor films 408 a and 408 c can bereduced even with a small amount of oxygen released from the insulatingfilm 411 b.

Note that when the ratio of the amount of the oxidizing gas to theamount of the deposition gas containing silicon is 100 or higher, thehydrogen content in the insulating film 411 a can be reduced.Consequently, the amount of hydrogen entering the oxide semiconductorfilms 408 a and 408 c can be reduced, inhibiting the negative shift inthe threshold voltage of the transistor.

As the insulating film 411 b, a silicon oxide film or a siliconoxynitride film is formed under the following conditions: the substrateplaced in a treatment chamber of a plasma CVD apparatus that isvacuum-evacuated is held at a temperature higher than or equal to 180°C. and lower than or equal to 280° C., preferably higher than or equalto 200° C. and lower than or equal to 240° C., the pressure in thetreatment chamber is greater than or equal to 100 Pa and less than orequal to 250 Pa, preferably greater than or equal to 100 Pa and lessthan or equal to 200 Pa with introduction of a source gas into thetreatment chamber, and a high-frequency power higher than or equal to0.17 W/cm² and lower than or equal to 0.5 W/cm², preferably higher thanor equal to 0.25 W/cm² and lower than or equal to 0.35 W/cm² is suppliedto an electrode provided in the treatment chamber.

A deposition gas containing silicon and an oxidizing gas are preferablyused as the source gas of the insulating film 411 b. Typical examples ofthe deposition gas containing silicon include silane, disilane,trisilane, and silane fluoride. Examples of the oxidizing gas includeoxygen, ozone, dinitrogen monoxide, and nitrogen dioxide.

As the film formation conditions for the insulating film 411 b, thehigh-frequency power having the above power density is supplied to thetreatment chamber having the above pressure, whereby the decompositionefficiency of the source gas in plasma is increased, oxygen radicals areincreased, and oxidation of the source gas is promoted; therefore, theoxygen content in the insulating film 411 b becomes higher than that inthe stoichiometric composition. On the other hand, when a substratetemperature is the above film formation temperature of the insulatingfilm 411 b, part of oxygen in the film is released by heat treatmentbecause of a weak bond between silicon and oxygen. Thus, it is possibleto form an oxide insulating film which contains oxygen at a higherproportion than the stoichiometric composition and from which part ofoxygen is released by heating. Furthermore, since the insulating film411 a is provided over the oxide semiconductor films 408 a and 408 c, inthe step of forming the insulating film 411 b, the insulating film 411 aserves as a protective film for the oxide semiconductor films 408 a and408 c. As a result, the insulating film 411 b can be formed using thehigh-frequency power having a high power density while damage to theoxide semiconductor films 408 a and 408 c is reduced.

Note that in the film formation conditions for the insulating film 411b, the flow rate of the deposition gas containing silicon relative tothe oxidizing gas can be increased, in which case the amount of defectsin the insulating film 411 b is reduced. Typically, it is possible toform an oxide insulating film in which the amount of defects is small,i.e., the spin density corresponding to a signal which appears atg=2.001 due to a dangling bond of silicon is lower than 6×10¹⁷spins/cm³, preferably lower than or equal to 3×10¹⁷ spins/cm³, and morepreferably lower than or equal to 1.5×10¹⁷ spins/cm³ by ESR measurement.As a result, the reliability of the transistor can be improved.

Next, heat treatment is performed. The heating temperature is typicallyhigher than or equal to 150° C. and lower than the strain point of thesubstrate, preferably higher than or equal to 200° C. and lower than orequal to 450° C., and more preferably higher than or equal to 300° C.and lower than or equal to 450° C. When the heat treatment is performedtypically at a temperature of 300° C. to 400° C., preferably 320° C. to370° C., warp or shrinking of a large-sized substrate can be reduced toimprove yield.

An electric furnace, an RTA apparatus, or the like can be used for theheat treatment. With the use of an RTA apparatus, the heat treatment canbe performed at a temperature higher than or equal to the strain pointof the substrate if the heating time is short. This leads to shorteningof the heat treatment time.

The heat treatment may be performed under an atmosphere of nitrogen,oxygen, ultra-dry air (air with a water content of 20 ppm or less,preferably 1 ppm or less, and more preferably 10 ppb or less), or a raregas (argon, helium, or the like). The atmosphere of nitrogen, oxygen,ultra-dry air, or a rare gas preferably does not contain hydrogen,water, and the like.

By the heat treatment, part of oxygen contained in the insulating film411 b can be moved to the oxide semiconductor films 408 a and 408 c,reducing oxygen vacancies contained in the oxide semiconductor films 408a and 408 c. Consequently, the amount of oxygen vacancies in the oxidesemiconductor films 408 a and 408 c can be further reduced.

In the case where water, hydrogen, or the like is contained in theinsulating films 411 a and 411 b, the water, hydrogen, or the likecontained in the insulating films 411 a and 411 b is moved to the oxidesemiconductor films 408 a and 408 c when the insulating film 413 havinga function of blocking water, hydrogen, and the like is formed later andheat treatment is performed, so that defects are generated in the oxidesemiconductor films 408 a and 408 c. However, by the heating, water,hydrogen, or the like contained in the insulating films 411 a and 411 bcan be released; thus, variation in electrical characteristics of thetransistor can be reduced and change in threshold voltage can beinhibited.

Note that when the insulating film 411 b is formed over the insulatingfilm 411 a while being heated, oxygen can be moved to the oxidesemiconductor films 408 a and 408 c and oxygen vacancies in the oxidesemiconductor films 408 a and 408 c can be reduced; thus, the heattreatment is not necessarily performed.

When the conductive films 410 a and 410 b are formed, the oxidesemiconductor films 408 a and 408 c are damaged by the etching of theconductive film, so that oxygen vacancies are generated on the backchannel side of the oxide semiconductor film 408 a (the side of theoxide semiconductor film 408 a that is opposite the side facing the gateelectrode 404). However, with the use of the oxide insulating filmcontaining oxygen at a higher proportion than the stoichiometriccomposition as the insulating film 411 b, the oxygen vacancies generatedon the back channel side can be repaired by heat treatment. This reducesdefects contained in the oxide semiconductor film 408 a to improve thereliability of the transistor.

Note that the heat treatment may be performed after the formation of theopening 462 which is formed later.

Then, the insulating film 411 is processed into desired regions, so thatthe insulating film 412 and the opening 462 are formed. The insulatingfilm 412 and the opening 462 can be formed in such a manner that a maskis formed in the desired regions by fourth patterning and regions notcovered with the mask are etched (see FIG. 8A).

The opening 462 is formed so as to expose the surface of the oxidesemiconductor film 408 c. An example of a formation method of theopening 462 includes, but not limited to, a dry etching method.Alternatively, a wet etching method or a combination of dry etching andwet etching can be employed for formation of the opening 462.

Next, the insulating film 413 is formed over the insulating film 406,the insulating film 412, and the oxide semiconductor film 408 c (seeFIG. 8B).

The insulating film 413 is preferably formed using a material that canprevent an external impurity such as oxygen, hydrogen, water, alkalimetal, or alkaline earth metal, from diffusing into the oxidesemiconductor film, more preferably formed using the material containinghydrogen, and typically an inorganic insulating material containingnitrogen, such as a nitride insulating film, can be used. The insulatingfilm 413 can be formed by a CVD method or the like.

The insulating film 413 is made of a material preventing diffusion ofimpurities from the outside, such as water, alkali metal, and alkalineearth metal, into the oxide semiconductor film, and further includeshydrogen. Therefore, when hydrogen in the insulating film 413 isdiffused into the oxide semiconductor film 408 c, hydrogen is bonded tooxygen and electrons serving as carriers are generated in the oxidesemiconductor film 408 c. When the insulating film 413 is formed by aplasma CVD method or a sputtering method, the oxide semiconductor filmis exposed to plasma and oxygen vacancies are generated in the oxidesemiconductor film. When hydrogen contained in the insulating film 414enters the oxygen vacancies, electrons serving as carriers aregenerated. As a result, the oxide semiconductor film 408 c has higherconductivity and becomes the conductive film 408 b.

The silicon nitride film is preferably formed at a high temperature tohave an improved blocking property; for example, the silicon nitridefilm is preferably formed at a substrate temperature of 100° C. to 400°C., more preferably at a temperature of 300° C. to 400° C. When thesilicon nitride film is formed at a high temperature, a phenomenon inwhich oxygen is released from the oxide semiconductor used for the oxidesemiconductor film 408 a and the carrier concentration is increased iscaused in some cases; therefore, the upper limit of the temperature is atemperature at which the phenomenon is not caused.

Then, the insulating films 413 and 412 are processed into desiredregions, so that the insulating film 414 and the opening 464 are formed.The insulating film 414 and the opening 464 can be formed in such amanner that a mask is formed in the desired regions by fifth patterningand regions not covered with the mask are etched (see FIG. 8C).

The opening 464 is formed so as to expose the surface of the conductivefilm 410 b.

An example of a formation method of the opening 464 includes, but notlimited to, a dry etching method. Alternatively, a wet etching method ora combination of dry etching and wet etching can be employed forformation of the opening 464.

Then, a conductive film 415 is formed over the insulating film 414 so asto cover the opening 464 (see FIG. 9A).

The conductive film 415 can be formed by a sputtering method or thelike.

Then, the conductive film 415 is processed into desired regions, so thatthe conductive film 416 is formed. The conductive film 416 can be formedin such a manner that a mask is formed in the desired regions by sixthpatterning and regions not covered with the mask are etched (see FIG.9B).

Through the above steps, the transistor 217 and the capacitor 219 can beformed over the substrate 402. Note that in the manufacturing process inthis embodiment, the transistor and the capacitor can be formed at thesame time by the first to sixth patterning, namely, with six masks.

In this embodiment, the conductivity of the oxide semiconductor film 408c is increased by diffusing hydrogen contained in the insulating film414 into the oxide semiconductor film 408 c; however, the conductivityof the oxide semiconductor film 408 c may be increased by covering theoxide semiconductor film 408 a with a mask and adding impurities,typically, hydrogen, boron, phosphorus, tin, antimony, a rare gaselement, alkali metal, alkaline earth metal, or the like to the oxidesemiconductor film 408 c. Hydrogen, boron, phosphorus, tin, antimony, arare gas element, or the like may be added to the oxide semiconductorfilm 408 c by an ion doping method, an ion implantation method, or thelike. Alkali metal, alkaline earth metal, or the like may be added tothe oxide semiconductor film 408 c by a method in which the oxidesemiconductor film 408 c is exposed to a solution containing theimpurity. Alternatively, the oxide semiconductor film 408 c may besubjected to treatment in a plasma atmosphere containing hydrogen andargon to introduce hydrogen.

Next, description is made on the element portion over the substrate 442facing the substrate 402. Note that the element portion over thesubstrate 442 refers to a region sandwiched between the substrate 442and the alignment film 452.

First, the substrate 442 is prepared. For materials of the substrate442, the materials that can be used for the substrate 402 can bereferred to. Then, the light-blocking film 444 and the coloring film 446are formed over the substrate 442 (see FIG. 10A).

The light-blocking film 444 and the coloring film 446 each are formed ina desired position with any of various materials by a printing method,an inkjet method, an etching method using a photolithography technique,or the like.

Then, the insulating film 448 is formed over the light-blocking film 444and the coloring film 446 (see FIG. 10B).

For the insulating film 448, an organic insulating film of an acrylicresin, an epoxy resin, polyimide, or the like can be used. With theinsulating film 448, an impurity or the like contained in the coloringfilm 446 can be prevented from diffusing into the liquid crystal layer420 side, for example. Note that the insulating film 448 is notnecessarily provided.

Next, the conductive film 450 is formed over the insulating film 448(see FIG. 10C). For materials of the conductive film 450, the materialsthat can be used for the conductive film 415 can be referred to.

Through the above steps, the structure over the substrate 442 can beobtained.

Next, the alignment film 418 is formed over the substrate 402,specifically, over the insulating film 414 and the conductive film 416formed over the substrate 402, and the alignment film 452 is formed overthe substrate 442, specifically, over the conductive film 450 formedover the substrate 442. The alignment films 418 and 452 can be formed bya rubbing method, an optical alignment method, or the like. After that,the liquid crystal layer 420 is formed between the substrate 402 and thesubstrate 442. The liquid crystal layer 420 can be formed by a dispensermethod (a dropping method), or an injecting method by which a liquidcrystal is injected using a capillary phenomenon after the substrate 402and the substrate 442 are bonded to each other.

With use of colored light as a backlight, a MEMS shutter may be drivenby a time-sequential method. In that case, the alignment film, theliquid crystal element, the coloring film, and the like over theconductive film 416 are not necessary.

Through the above steps, the semiconductor device illustrated in FIG. 5can be manufactured.

A structure in which the transistor 218 is provided below the transistor217 and the capacitor 219, which are semiconductor devices in thecontrol circuit 200, will be described with reference to FIG. 17. Thetransistor 218 is described in detail below.

FIG. 17 is a cross-sectional view of the structure in which thetransistor 218 is provided below the transistor 217 and the capacitor219. In a semiconductor device illustrated in FIG. 17, the transistor218 using a material other than an oxide semiconductor is provided inthe lower part, while the transistor 217 and the capacitor 219 using anoxide semiconductor are provided in the upper part. Although thetransistor 218 is an n-channel transistor here, it may be a p-channeltransistor. In particular, the transistor 218 can easily have p-typeconductivity.

As illustrated in FIG. 17, the transistor 218 is formed on a substrate550. The substrate 550 can be similar to the substrate 402.

The transistor 218 is electrically isolated from another transistor byan element isolation insulating film 551. The element isolationinsulating film 551 can be formed by a local oxidation of silicon(LOCOS) method, a trench isolation method, or the like. Note that asilicon on insulator (SOI) type semiconductor substrate may be used asthe substrate 550. In that case, a semiconductor layer may be divided byetching into elements for isolation.

The transistor 218 includes a high-concentration impurity region 557, alow-concentration impurity region 558, a gate electrode 559, and a gateinsulating film 556 provided between the substrate 550 and the gateelectrode 559. A sidewall insulating film 587 is formed around the gateelectrode 559.

An insulating film 566 is formed on the transistor 218. The insulatingfilm 566 includes an opening, and a wiring 562 and a wiring 563 areformed in the opening so as to be in contact with the high-concentrationimpurity region 557. A wiring 565 is formed in contact with the gateelectrode 559.

The wiring 562 is electrically connected to a wiring 568 formed over theinsulating film 566, the wiring 563 is electrically connected to awiring 570 formed over the insulating film 566, and the wiring 565 iselectrically connected to a wiring 569 formed over the insulating film566.

An insulating film 571 is formed over the wirings 568 to 570. Thetransistor 217 including an oxide semiconductor is formed over theinsulating film 571. The insulating film 571 includes an opening, and inthe opening, the wiring 569 is electrically connected to one of theconductive films 410 a and 410 b serving as a source electrode and adrain electrode of the transistor 217.

As illustrated in FIG. 18, the transistor 218 may include an oxidesemiconductor in a channel region. The oxide semiconductor of thetransistor 218 may be a CAAC-OS film, and the oxide semiconductor of thetransistor 217 may be a microcrystalline oxide semiconductor film. Notethat the materials of the transistor 217 described above can be referredto for the materials of an oxide semiconductor film 508, conductivefilms 510 a and 510 b serving as a source electrode and a drainelectrode, a gate insulating film 505, a gate electrode 504, and thelike of the transistor 218 including an oxide semiconductor.

When the transistor 217 and the capacitor 219 are thus stacked over thetransistor 218, a miniaturized display device occupying a small area canbe manufactured.

This embodiment can be combined with any of the other embodiments inthis specification as appropriate.

Embodiment 2

Described in this embodiment are modified examples of the transistor 217and the capacitor 219 shown in Embodiment 1.

Modified Example 1 Transistor

In FIG. 11A, the oxide semiconductor film 408 a of the transistor 217shown in Embodiment 1 is connected to the conductive films 410 a and 410b in a manner different from that illustrated in FIG. 5. This modifiedexample shows a bottom-contact transistor 257.

A transistor 267 illustrated in FIG. 11B does not include the conductivefilms 410 a and 410 b which serves as the source electrode and the drainelectrode of the transistor 217 shown in Embodiment 1. Instead, openingsare formed in the insulating films 412 and 414, and the conductive film416 and a conductive film 417 serving as a source electrode and a drainelectrode are formed so as to be in contact with the oxide semiconductorfilm 408 a through the openings. Note that the conductive film 416 alsoserves as one electrode of the capacitor 219.

Modified Example 2 Base Insulating Film

In the transistor 217 described in Embodiment 1, a base insulating filmcan be provided between the substrate 402 and the gate electrode 404 asnecessary. Examples of a material of the base insulating film includesilicon oxide, silicon oxynitride, silicon nitride, silicon nitrideoxide, gallium oxide, hafnium oxide, yttrium oxide, aluminum oxide, andaluminum oxynitride. Note that when silicon nitride, gallium oxide,hafnium oxide, yttrium oxide, aluminum oxide, or the like is used forthe base insulating film, it is possible to suppress diffusion ofimpurities such as alkali metal, water, and hydrogen into the oxidesemiconductor film 408 a from the substrate 402.

The base insulating film can be formed by a sputtering method, a CVDmethod, or the like.

Modified Example 3 Gate Insulating Film

In the transistor 217 described in Embodiment 1, the layered structureof the insulating film serving as the gate insulating film can bechanged as necessary.

As illustrated in FIG. 12A, the gate insulating film has a layeredstructure in which the insulating film 405 and the insulating film 406are stacked in this order from the gate electrode 404 side.

When the insulating film 405 formed using a nitride insulating film isprovided on the gate electrode 404 side, an impurity, typicallyhydrogen, nitrogen, alkali metal, alkaline earth metal, or the like, canbe prevented from moving from the gate electrode 404 to the oxidesemiconductor film 408 a.

Furthermore, the insulating film 406 formed using an oxide insulatingfilm is provided on the oxide semiconductor film 408 a side, therebyreducing the density of defect states at the interface between theinsulating film 406 and the oxide semiconductor film 408 a.Consequently, a transistor whose electrical characteristics are hardlydegraded can be obtained. Note that like the insulating film 412 b, theinsulating film 406 is more preferably formed using an oxide insulatingfilm containing oxygen at a higher proportion than the stoichiometriccomposition, in which case the density of defect states at the interfacebetween the insulating film 406 and the oxide semiconductor film 408 acan be further reduced.

As illustrated in FIG. 12A, the insulating film 405 can have a layeredstructure in which a nitride insulating film 405 a with few defects anda nitride insulating film 405 b with a high blocking property againsthydrogen are stacked in this order from the gate electrode 404 side.When the nitride insulating film 405 a with few defects is provided inthe gate insulating film 405, the withstand voltage of the gateinsulating film can be improved. In addition, when the nitrideinsulating film 405 b with a high blocking property against hydrogen isprovided, hydrogen can be prevented from moving from the gate electrode404 and the nitride insulating film 405 a to the oxide semiconductorfilm 408 a.

An example of a method for manufacturing the nitride insulating films405 a and 405 b illustrated in FIG. 12A will be described below. First,as the nitride insulating film 405 a, a silicon nitride film with fewdefects is formed by a plasma CVD method using a mixed gas of silane,nitrogen, and ammonia as a source gas. Then, as the nitride insulatingfilm 405 b, a silicon nitride film which has a low hydrogenconcentration and can block hydrogen is formed by changing the sourcegas to a mixed gas of silane and nitrogen. Such a formation method makesit possible to form the gate insulating film having a stack of nitrideinsulating films with few defects and a blocking property againsthydrogen.

Alternatively, as illustrated in FIG. 12B, the insulating film 405 canhave a layered structure in which a nitride insulating film 405 c with ahigh blocking property against an impurity, the nitride insulating film405 a with few defects, and the nitride insulating film 405 b with ahigh blocking property against hydrogen are stacked in this order fromthe gate electrode side. When the nitride insulating film 405 c with ahigh blocking property against an impurity is provided in the insulatingfilm 405, an impurity, typically hydrogen, nitrogen, alkali metal,alkaline earth metal, or the like, can be prevented from moving from thegate electrode to the oxide semiconductor film 408 a.

An example of a method for manufacturing the nitride insulating films405 a, 405 b, and 405 c illustrated in FIG. 12B will be described below.First, as the nitride insulating film 405 c, a silicon nitride film witha high blocking property against an impurity is formed by a plasma CVDmethod using a mixed gas of silane, nitrogen, and ammonia as a sourcegas. Next, a silicon nitride film with few defects is formed as thenitride insulating film 405 a by increasing the flow rate of ammonia.Then, as the nitride insulating film 405 b, a silicon nitride film whichhas a low hydrogen concentration and can block hydrogen is formed bychanging the source gas to a mixed gas of silane and nitrogen. Such aformation method makes it possible to form the insulating film 405having a stack of nitride insulating films with few defects and ablocking property against an impurity.

Modified Example 4 A Pair of Electrodes

Description is made on the materials for the conductive films 410 a and410 b of the transistor 217 shown in Embodiment 1.

For the conductive films 410 a and 410 b provided in the transistor 217shown in Embodiment 1, it is preferable to use a conductive materialwhich is easily bonded to oxygen, such as tungsten, titanium, aluminum,copper, molybdenum, chromium, or tantalum, or an alloy thereof. As aresult, oxygen contained in the oxide semiconductor film 408 a is bondedto the conductive material contained in the conductive films 410 a and410 b, so that an oxygen deficient region is formed in the oxidesemiconductor film 408 a. In some cases, part of constituent elements ofthe conductive material that forms the conductive films 410 a and 410 bis mixed into the oxide semiconductor film 408 a. Consequently, asillustrated in FIG. 13, low-resistance regions 434 a and 434 b areformed in the vicinity of regions of the oxide semiconductor film 408 awhich are in contact with the conductive films 410 a and 410 b. Thelow-resistance regions 434 a and 434 b are formed between the insulatingfilm 406 and the conductive films 410 a and 410 b so as to be in contactwith the conductive films 410 a and 410 b. Since the low-resistanceregions 434 a and 434 b have high conductivity, contact resistancebetween the oxide semiconductor film 408 a and the conductive films 410a and 410 b can be reduced, increasing the on-state current of thetransistor.

The conductive films 410 a and 410 b may each have a layered structureof the conductive material which is easily bonded to oxygen and aconductive material which is not easily bonded to oxygen, such astitanium nitride, tantalum nitride, or ruthenium. Such a layeredstructure prevents oxidization of the conductive films 410 a and 410 bat the interface between the conductive films 410 a and 410 b and theoxide semiconductor film 408 a, thereby inhibiting an increase in theresistance of the conductive films 410 a and 410 b.

Modified Example 5 Oxide Semiconductor Film

In the method for manufacturing the transistor 217 described inEmbodiment 1, after the conductive films 410 a and 410 b are formed, theoxide semiconductor film 408 a may be exposed to plasma generated in anoxygen atmosphere, so that oxygen may be supplied to the oxidesemiconductor film 408 a. Examples of the oxidizing gas include oxygen,ozone, dinitrogen monoxide, and nitrogen dioxide. Furthermore, in theplasma treatment, the oxide semiconductor film 408 a is preferablyexposed to plasma generated with no bias applied to the substrate 402side. Consequently, the oxide semiconductor film 408 a can be suppliedwith oxygen without being damaged, resulting in a reduction in theamount of oxygen vacancies in the oxide semiconductor film 408 a.Moreover, impurities, e.g., halogen such as fluorine or chlorineremaining on the surface of the oxide semiconductor film 408 a due tothe etching treatment can be removed.

Modified Example 6 Oxide Semiconductor Film

In the transistor 217 described in Embodiment 1, the oxide semiconductorfilm can have a layered structure as necessary.

In the transistor illustrated in FIG. 14, a multilayer film 436including an oxide semiconductor film is formed between the insulatingfilm 406 and the conductive films 410 a and 410 b.

The multilayer film 436 includes an oxide semiconductor film 436 a andan oxide film 436 b. That is, the multilayer film 436 has a two-layerstructure. Part of the oxide semiconductor film 436 a serves as achannel region. Furthermore, the insulating film 412 a is formed incontact with the multilayer film 436, and the oxide film 436 b is formedin contact with the insulating film 412 a. That is, the oxide film 436 bis provided between the oxide semiconductor film 436 a and theinsulating film 412 a.

The oxide film 436 b contains one or more elements which form the oxidesemiconductor film 436 a. Since the oxide film 436 b contains one ormore elements which form the oxide semiconductor film 436 a, interfacescattering is unlikely to occur at the interface between the oxidesemiconductor film 436 a and the oxide film 436 b. Thus, the transistorcan have a high field-effect mobility because the movement of carriersis not hindered at the interface.

The oxide film 436 b is typically In—Ga oxide, In—Zn oxide, or In—M-Znoxide (M is Al, Ti, Ga, Y, Zr, La, Ce, Nd, or Hf). The energy at theconduction band bottom of the oxide film 436 b is closer to a vacuumlevel than that of the oxide semiconductor film 436 a is, and typically,the difference between the energy at the conduction band bottom of theoxide film 436 b and the energy at the conduction band bottom of theoxide semiconductor film 436 a is any one of 0.05 eV or more, 0.07 eV ormore, 0.1 eV or more, and 0.15 eV or more, and any one of 2 eV or less,1 eV or less, 0.5 eV or less, and 0.4 eV or less. That is, thedifference between the electron affinity of the oxide film 436 b and theelectron affinity of the oxide semiconductor film 436 a is any one of0.05 eV or more, 0.07 eV or more, 0.1 eV or more, and 0.15 eV or more,and any one of 2 eV or less, 1 eV or less, 0.5 eV or less, and 0.4 eV orless.

The oxide film 436 b preferably contains In because carrier mobility(electron mobility) can be increased.

When the oxide film 436 b contains a larger amount of Al, Ti, Ga, Y, Zr,La, Ce, Nd, or Hf in an atomic ratio than the amount of In in an atomicratio, the following effects can be obtained in some cases: (1) increasein the energy gap of the oxide film 436 b; (2) decrease in the electronaffinity of the oxide film 436 b; (3) blocking of an impurity from theoutside; (4) higher insulating property than that of the oxidesemiconductor film 436 a; and (5) less oxygen vacancies in the oxidefilm 436 b containing a larger amount of Al, Ti, Ga, Y, Zr, La, Ce, Nd,or Hf in an atomic ratio than the amount of In in an atomic ratiobecause Al, Ti, Ga, Y, Zr, La, Ce, Nd, or Hf is a metal element which isstrongly bonded to oxygen.

When the oxide film 436 b includes an In—M-Zn oxide, the proportion ofIn and the proportion of M, not taking Zn and O into consideration, arepreferably less than 50 atomic % and greater than or equal to 50 atomic%, respectively, more preferably less than 25 atomic % and greater thanor equal to 75 atomic %, respectively.

In the case where each of the oxide semiconductor film 436 a and theoxide film 436 b is In—M-Zn oxide film (M is Al, Ti, Ga, Y, Zr, La, Ce,Nd, or Hf), the proportion of M atoms (M is Al, Ti, Ga, Y, Zr, La, Ce,Nd, or Hf) in the oxide film 436 b is higher than that in the oxidesemiconductor film 436 a. Typically, the proportion of Min the oxidefilm 436 b is 1.5 or more times, preferably twice or more, and morepreferably three or more times as high as that in the oxidesemiconductor film 436 a.

In the case where the oxide semiconductor film 436 a is an In—M-Zn oxide(M is Al, Ti, Ga, Y, Zr, La, Ce, Nd, or Hf, and In:M:Zn=x₁:y₁:z₁ [atomicratio]) and the oxide film 436 b is an In—M-Zn oxide (M is Al, Ti, Ga,Y, Zr, La, Ce, Nd, or Hf, and In:M:Zn=x₂:y₂:z₂ [atomic ratio]), y₁/x₁ isgreater than y₂/x₂, or preferably y₁/x₁ is 1.5 or more times as much asy₂/x₂. More preferably, y₁/x₁ is twice or more as much as y₂/x₂, orstill more preferably y₁/x₁ is three or more times as much as y₂/x₂. Inthis case, it is preferable that in the oxide semiconductor film, y₂ begreater than or equal to x₂ because a transistor including the oxidesemiconductor film can have stable electrical characteristics. However,when y₂ is larger than or equal to three times x₂, the field-effectmobility of the transistor including the oxide semiconductor film isreduced. Accordingly, y₂ is preferably smaller than three times x₂.

In the case where the oxide semiconductor film 436 a is an In—M-Zn oxidefilm and a target having the atomic ratio of metal elements ofIn:M:Zn=x₁:y₁:z₁ is used for forming the oxide semiconductor film 436 a,x₁/y₁ is preferably greater than or equal to 1/3 and less than or equalto 6, more preferably greater than or equal to 1 and less than or equalto 6, and z₁/y₁ is preferably greater than or equal to 1/3 and less thanor equal to 6, more preferably greater than or equal to 1 and less thanor equal to 6. Note that when z₁/y₁ is greater than or equal to 1 andless than or equal to 6, a CAAC-OS film is easily formed as the oxidesemiconductor film 436 a. Typical examples of the atomic ratio of themetal elements of the target are In:M:Zn=1:1:1 and In:M:Zn=3:1:2.

In the case where the oxide film 436 b is an In—M-Zn oxide film, when atarget used for forming the oxide film 436 b has an atomic ratio ofmetal elements of In:M:Zn=x₂:y₂:z₂, x₂/y₂<x₁/y₁ is satisfied and z₂/y₂is preferably greater than or equal to 1/3 and less than or equal to 6,more preferably greater than or equal to 1 and less than or equal to 6.Note that when z₂/y₂ is greater than or equal to 1 and less than orequal to 6, a CAAC-OS film is easily formed as the oxide film 436 b.Typical examples of the atomic ratio of the metal elements of the targetare In:M:Zn=1:3:2 and In:M:Zn=1:3:3.

The oxide film 436 b also serves as a film which relieves damage to theoxide semiconductor film 436 a at the time of forming the insulatingfilm 412 b later.

The oxide film 436 b has a thickness of 3 nm to 100 nm, preferably 3 nmto 50 nm.

The oxide film 436 b may have a non-single-crystal structure like theoxide semiconductor film 436 a, for example. The non-single crystalstructure includes a c-axis aligned crystalline oxide semiconductor(CAAC-OS), a polycrystalline structure, a microcrystalline structure, oran amorphous structure, for example.

Note that the oxide semiconductor film 436 a and the oxide film 436 bmay each be a mixed film including two or more of the following: aregion having an amorphous structure, a region having a microcrystallinestructure, a region having a polycrystalline structure, a CAAC-OSregion, and a region having a single-crystal structure. In some cases,the mixed film has a layered structure of two or more of a region havingan amorphous structure, a region having a microcrystalline structure, aregion having a polycrystalline structure, a CAAC-OS region, and aregion having a single-crystal structure.

Here, the oxide film 436 b is provided between the oxide semiconductorfilm 436 a and the insulating film 412 a. Hence, if trap states areformed between the oxide film 436 b and the insulating film 412 a owingto impurities and defects, electrons flowing in the oxide semiconductorfilm 436 a are less likely to be captured by the trap states becausethere is a distance between the trap states and the oxide semiconductorfilm 436 a. Accordingly, the amount of on-state current of thetransistor can be increased, and the field-effect mobility can beincreased. When the electrons are captured by the trap states, theelectrons become negative fixed charges. As a result, the thresholdvoltage of the transistor varies. However, the distance between theoxide semiconductor film 436 a and the trap states reduces capture ofthe electrons by the trap states, and accordingly reduces variation inthreshold voltage.

Impurities from the outside can be blocked by the oxide film 436 b,which results in a reduction in the amount of impurities moving from theoutside to the oxide semiconductor film 436 a. In addition, an oxygenvacancy is less likely to be formed in the oxide film 436 b. It is thuspossible to reduce the impurity concentration and the amount of oxygenvacancies in the oxide semiconductor film 436 a.

Note that the oxide semiconductor film 436 a and the oxide film 436 bare not formed by simply stacking each film, but are formed to form acontinuous junction (here, in particular, a structure in which theenergy of the bottom of the conduction band is changed continuouslybetween each film). In other words, a layered structure is formed suchthat there exist no impurities forming a defect state such as a trapcenter or a recombination center at the interface between the oxidesemiconductor film 436 a and the oxide film 436 b. If an impurity existsbetween the oxide semiconductor film 436 a and the oxide film 436 bwhich are stacked, the continuity of the energy band is damaged, and thecarrier is captured or recombined at the interface and then disappears.

To form the continuous junction, each film needs to be stackedsuccessively without exposure to the atmosphere using a multi-chamberdeposition apparatus (sputtering apparatus) including a load lockchamber. Each chamber in the sputtering apparatus is preferablysubjected to high vacuum evacuation (to a vacuum of about 5×10⁻⁷ Pa to1×10⁻⁴ Pa) with use of a suction vacuum evacuation pump such as acryopump so that water or the like, which is an impurity for the oxidesemiconductor film, is removed as much as possible. Alternatively, aturbo-molecular pump is preferably used in combination with a cold trapto prevent backflow of a gas, particularly a gas containing carbon orhydrogen into the chamber through an evacuation system.

In FIG. 14, the multilayer film 436 has a two-layer structure of theoxide semiconductor film 436 a and the oxide film 436 b; however themultilayer film 436 may have a three-layer structure in which a filmsimilar to the oxide film 436 b is further provided between theinsulating film 406 and the oxide semiconductor film 436 a. In thiscase, the thickness of the oxide film provided between the insulatingfilm 406 and the oxide semiconductor film 436 a is preferably less thanthat of the oxide semiconductor film 436 a. When the thickness of theoxide film is greater than or equal to 1 nm and less than or equal to 5nm, preferably greater than or equal to 1 nm and less than or equal to 3nm, the amount of change in the threshold voltage of the transistor canbe reduced.

Modified Example 7 Oxide Semiconductor Film

The structure of the multilayer film including the oxide semiconductorfilm shown in modified example 6 can be changed as appropriate.

As illustrated in FIG. 15, the multilayer film 436 including an oxidesemiconductor film is formed between the insulating film 406 and theinsulating film 412 a.

The multilayer film 436 includes the oxide semiconductor film 436 aformed between the insulating film 406 and the conductive films 410 aand 410 b, and the oxide film 436 b formed over the oxide semiconductorfilm 436 a and the conductive films 410 a and 410 b. Part of the oxidesemiconductor film 436 a serves as a channel region. Furthermore, theinsulating film 412 a is formed in contact with the multilayer film 436,and the oxide film 436 b is formed in contact with the insulating film412 a. That is, the oxide film 436 b is provided between the oxidesemiconductor film 436 a and the insulating film 412 a.

The transistor 217 shown in this modified example has a lower contactresistance between the oxide semiconductor film 436 a and the conductivefilms 410 a and 410 b and an increased on-state current as compared tothe transistor in modified example 6 because the conductive films 410 aand 410 b are in contact with the oxide semiconductor film 436 a.

Furthermore, since the conductive films 410 a and 410 b are in contactwith the oxide semiconductor film 436 a in the transistor 217 in thismodified example, the thickness of the oxide film 436 b can be increasedwithout increase of the contact resistance between the oxidesemiconductor film 436 a and the conductive films 410 a and 410 b. Thus,it is possible to inhibit formation of a trap state, which occurs due toplasma damage at the time of forming the insulating film 412 b, mixingof a constituent element of the insulating films 412 a and 412 b, or thelike, in the vicinity of the interface between the oxide semiconductorfilm 436 a and the oxide film 436 b. That is, the transistor in thismodified example can achieve both improvement of on-state current andreduction in variation in threshold voltage.

Modified Example 8 Structure of Transistor

The transistor 217 described in Embodiment 1 can include a plurality ofgate electrodes facing each other with an oxide semiconductor filminterposed therebetween as necessary.

The transistor 217 illustrated in FIG. 16 includes the gate electrode404 over the substrate 402. The transistor 217 also includes theinsulating films 405 and 406 formed over the substrate 402 and the gateelectrode 404, the oxide semiconductor film 408 a overlapping with thegate electrode 404 with the insulating films 405 and 406 interposedtherebetween, and the conductive films 410 a and 410 b in contact withthe oxide semiconductor film 408 a. Over the insulating film 406, theoxide semiconductor film 408 a, and the conductive films 410 a and 410b, the insulating film 412 including a stack of the insulating films 412a and 412 b, and the insulating film 414 are formed. A conductive film456 is provided to overlap with the oxide semiconductor film 408 a withthe insulating films 412 and 414 interposed therebetween.

The gate electrode 404 faces the conductive film 456 with the oxidesemiconductor film 408 a interposed therebetween. The conductive film456 serves as a gate electrode. The conductive film 456 is preferablyformed at the same time as the conductive film 416 to reduce the numberof manufacturing steps.

The transistor 217 shown in this modified example includes the gateelectrode 404 and the conductive film 456 which face each other with theoxide semiconductor film 408 a interposed therebetween. The thresholdvoltage of the transistor 217 can be controlled by supplying differentpotentials to the gate electrode 404 and the conductive film 456.

The structures, methods, and the like described in this embodiment canbe used as appropriate in combination with any of the structures,methods, and the like described in the other embodiments.

Embodiment 3

The semiconductor device of one embodiment of the present invention canbe used in a sensor that can detect proximity or touch of an object(e.g., a capacitive, a resistive, a surface acoustic wave, an infrared,and an optical touch sensor) and a radiographic image detection devicethat can obtain a medical radiographic image. The semiconductor deviceof one embodiment of the present invention can also be applied to avariety of electronic devices (including game machines). Examples ofelectronic devices include a television device (also referred to astelevision or television receiver), a monitor of a computer or the like,a digital camera, a digital video camera, a digital photo frame, amobile phone, a portable game machine, a portable information terminal,an audio reproducing device, a game machine (e.g., a pachinko machine ora slot machine), and a game console. Examples of these electronicdevices are illustrated in FIGS. 19A to 19C.

FIG. 19A illustrates a table 9000 including a display portion. In thetable 9000, a display portion 9003 is incorporated in a housing 9001 andan image can be displayed on the display portion 9003. The housing 9001is supported by four leg portions 9002. The housing 9001 also includes apower cord 9005 for supplying power.

The semiconductor device described in any of the above embodiments canbe used for the display portion 9003; therefore, the display portion9003 can have high display quality.

The display portion 9003 has a touch-input function. When a user touchesdisplayed buttons 9004 which are displayed on the display portion 9003of the table 9000 with his/her fingers or the like, the user can carryout operation of the screen and input of information. Furthermore, thetable 9000 may be made to communicate with home appliances or controlthe home appliances, in which case the table 9000 may function as acontrol device which controls the home appliances by operation on thescreen. For example, with use of a semiconductor device having an imagesensor function, the display portion 9003 can have a touch-inputfunction.

The screen of the display portion 9003 can also be placed perpendicularto a floor with a hinge provided for the housing 9001, in which case thetable 9000 can also be used as a television device. When a televisiondevice having a large screen is set in a small room, an open space isreduced; however, when a display portion is incorporated in a table, aspace in the room can be efficiently used.

FIG. 19B illustrates a television device 9100. In the television device9100, a display portion 9103 is incorporated in a housing 9101 and animage can be displayed on the display portion 9103. Note that thehousing 9101 is supported by a stand 9105 here.

The television device 9100 can be operated with an operation switch ofthe housing 9101 or a separate remote controller 9110. Channels andvolume can be controlled with an operation key 9109 of the remotecontroller 9110 so that an image displayed on the display portion 9103can be controlled. Furthermore, the remote controller 9110 may beprovided with a display portion 9107 for displaying data output from theremote controller 9110.

The television device 9100 illustrated in FIG. 19B includes a receiver,a modem, and the like. With the use of the receiver, the televisiondevice 9100 can receive general TV broadcasts. Moreover, when thetelevision device 9100 is connected to a communication network with orwithout wires via the modem, one-way (from a sender to a receiver) ortwo-way (between a sender and a receiver or between receivers)information communication can be performed.

The semiconductor device described in any of the above embodiments canbe used in the display portions 9103 and 9107; therefore, the televisiondevice can have high display quality.

FIG. 19C illustrates a computer 9200, which includes a main body 9201, ahousing 9202, a display portion 9203, a keyboard 9204, an externalconnection port 9205, a pointing device 9206, and the like.

The semiconductor device described in any of the above embodiments canbe used for the display portion 9203; therefore, the computer 9200 canhave high display quality.

FIGS. 20A and 20B illustrate a tablet terminal that can be folded. InFIG. 20A, the tablet terminal is opened, and includes a housing 9630, adisplay portion 9631 a, a display portion 9631 b, a display-modeswitching button 9034, a power button 9035, a power-saving-modeswitching button 9036, a clip 9033, and an operation button 9038.

The semiconductor device described in any of the above embodiments canbe used for the display portion 9631 a and the display portion 9631 b;therefore, the tablet terminal can have high display quality.

Part of the display portion 9631 a can be a touch panel region 9632 a,and data can be input by touching operation keys 9638 displayed.Although the display portion 9631 a having a structure in which a halfregion in the display portion 9631 a has only a display function and theother half region also has a touch panel function is illustrated as anexample, the structure of the display portion 9631 a is not limitedthereto. The whole display portion 9631 a may have a touch panelfunction. For example, a keyboard is displayed on the whole displayportion 9631 a so that the display portion 9631 a serves as a touchpanel, and the display portion 9631 b can be used as a display screen.

As in the display portion 9631 a, part of the display portion 9631 b canbe a touch panel region 9632 b. When a keyboard display switching button9639 displayed on the touch panel is touched with a finger, a stylus, orthe like, a keyboard can be displayed on the display portion 9631 b.

Touch input can be performed in the touch panel region 9632 a and thetouch panel region 9632 b at the same time.

The display-mode switching button 9034 can switch the display betweenportrait mode, landscape mode, and the like, and between monochromedisplay and color display, for example. With the button 9036 forswitching to power-saving mode, the luminance of display can beoptimized in accordance with the amount of external light at the timewhen the tablet terminal is in use, which is detected with an opticalsensor incorporated in the tablet terminal. The tablet terminal mayinclude another detection device such as a sensor for determiningorientation (e.g., a gyroscope or an acceleration sensor) in addition tothe optical sensor.

Although the display portion 9631 a and the display portion 9631 b havethe same display area in FIG. 20A, one embodiment of the presentinvention is not limited to this structure. The display portion 9631 aand the display portion 9631 b may have different areas or differentdisplay quality. For example, one of them may display higher-definitionimages than the other.

The tablet terminal is closed in FIG. 20B. The tablet terminal includesthe housing 9630, a solar cell 9633, and a charge and discharge controlcircuit 9634. FIG. 20B shows a structure in which the charge anddischarge control circuit 9634 includes a battery 9635 and a DCDCconverter 9636.

Since the tablet terminal is foldable, the housing 9630 can be closedwhen the tablet terminal is not used. As a result, the display portion9631 a and the display portion 9631 b can be protected, which offers atablet terminal having excellent durability and high reliability interms of long-term use.

In addition, the tablet terminal illustrated in FIGS. 20A and 2B canhave a function of displaying a variety of kinds of data (e.g., a stillimage, a moving image, and a text image), a function of displaying acalendar, a date, the time, or the like on the display portion, atouch-input function of operating or editing the data displayed on thedisplay portion by touch input, a function of controlling processing bya variety of kinds of software (programs), and the like.

The solar cell 9633 provided on a surface of the tablet terminal cansupply power to the touch panel, the display portion, a video signalprocessing portion, or the like. Note that the solar cell 9633 can beprovided on one or both surfaces of the housing 9630 and the battery9635 can be charged efficiently. The use of a lithium ion battery as thebattery 9635 is advantageous in downsizing or the like.

The structure and operation of the charge and discharge control circuit9634 illustrated in FIG. 20B will be described with reference to a blockdiagram in FIG. 20C.

FIG. 20C illustrates the solar cell 9633, the battery 9635, the DCDCconverter 9636, a converter 9637, switches SW1 to SW3, and the displayportion 9631. The battery 9635, the DCDC converter 9636, the converter9637, and the switches SW1 to SW3 correspond to the charge and dischargecontrol circuit 9634 illustrated in FIG. 20B.

First, description is made on an example of the operation in the casewhere power is generated by the solar cell 9633 using external light.The voltage of power generated by the solar cell 9633 is stepped up ordown by the DCDC converter 9636 so that the power has a voltage forcharging the battery 9635. Then, when the power from the solar cell 9633is used for the operation of the display portion 9631, the switch SW1 isturned on and the voltage of the power is stepped up or down by theconverter 9637 so as to be a voltage needed for the display portion9631. When display on the display portion 9631 is not performed, theswitch SW1 is turned off and the switch SW2 is turned on so that thebattery 9635 can be charged.

Note that the solar cell 9633 is described as an example of a powergeneration means; however, without limitation thereon, the battery 9635may be charged using another power generation means such as apiezoelectric element or a thermoelectric conversion element (Peltierelement). For example, the battery 9635 may be charged with anon-contact power transmission module which is capable of charging bytransmitting and receiving power by wireless (without contact), oranother charge means used in combination.

The structures, methods, and the like described in this embodiment canbe used as appropriate in combination with any of the structures,methods, and the like described in the other embodiments.

This application is based on Japanese Patent Application serial No.2013-088181 filed with Japan Patent Office on Apr. 19, 2013, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A display device comprising: a display portioncomprising a pixel; and a control circuit comprising: a light-blockingunit comprising a first layer and a second layer; a first transistorelectrically connected to the light-blocking unit; and a capacitorelectrically connected to the first transistor, wherein: the first layerhas a first opening overlapping with at least part of the pixel, thesecond layer is configured to block light passing through the firstopening, the first transistor comprises an oxide semiconductor film, thecapacitor comprises a first conductive film, and the oxide semiconductorfilm and the first conductive film are on a same surface.
 2. The displaydevice according to claim 1, the first transistor further comprising: agate electrode; a gate insulating film in contact with the gateelectrode, the oxide semiconductor film, and the first conductive film;and a pair of conductive films in contact with the oxide semiconductorfilm.
 3. The display device according to claim 1, the control circuitfurther comprising: a first insulating film covering the firsttransistor, and the capacitor further comprising: a second insulatingfilm over the first insulating film; and a second conductive film overthe second insulating film, wherein: the first insulating film has asecond opening on the first conductive film, the second insulating filmis in contact with the first conductive film in the second opening, andthe second conductive film is electrically connected to the firsttransistor.
 4. The display device according to claim 3, wherein thefirst insulating film is an oxide insulating film, and wherein thesecond insulating film is a nitride insulating film.
 5. The displaydevice according to claim 1, wherein each of the oxide semiconductorfilm and the first conductive film comprises at least one of In, Ga, andZn.
 6. The display device according to claim 1, wherein thelight-blocking unit is a MEMS shutter.
 7. The display device accordingto claim 1, further comprising: a second transistor, wherein: the secondtransistor overlaps with the first transistor and the capacitor, thesecond transistor is electrically connected to the first transistor andthe capacitor, and the second transistor is on a substrate including asemiconductor material.
 8. A display device comprising: a displayportion comprising a pixel; and a control circuit comprising: alight-blocking unit comprising a first layer, a second layer, and anactuator; a first transistor electrically connected to thelight-blocking unit; and a capacitor electrically connected to the firsttransistor, wherein: the first layer has a first opening overlappingwith at least part of the pixel, the second layer is configured to blocklight passing through the first opening, the actuator comprises amovable electrode electrically connected to the second layer, theactuator is configured to move the second layer, the first transistorcomprises an oxide semiconductor film, the capacitor comprises a firstconductive film, and the oxide semiconductor film and the firstconductive film are on a same surface.
 9. The display device accordingto claim 8, the first transistor further comprising: a gate electrode; agate insulating film in contact with the gate electrode, the oxidesemiconductor film, and the first conductive film; and a pair ofconductive films in contact with the oxide semiconductor film.
 10. Thedisplay device according to claim 8, the control circuit furthercomprising: a first insulating film covering the first transistor, andthe capacitor further comprising: a second insulating film over thefirst insulating film; and a second conductive film over the secondinsulating film, wherein: the first insulating film has a second openingon the first conductive film, the second insulating film is in contactwith the first conductive film in the second opening, and the secondconductive film is electrically connected to the first transistor. 11.The display device according to claim 10, wherein the first insulatingfilm is an oxide insulating film, and wherein the second insulating filmis a nitride insulating film.
 12. The display device according to claim8, wherein each of the oxide semiconductor film and the first conductivefilm comprises at least one of In, Ga, and Zn.
 13. The display deviceaccording to claim 8, further comprising: a second transistor, wherein:the second transistor overlaps with the first transistor and thecapacitor, the second transistor is electrically connected to the firsttransistor and the capacitor, and the second transistor is on asubstrate including a semiconductor material.